Blame for backend/billing.py

"""
Razorpay billing blueprint for LarShield.

Checkout model
--------------
One-time payment (mode: order). Each successful purchase unlocks
SCANS_PER_PURCHASE scans for the buyer's organization.

Pricing model
-------------
The customer pays the plan price *plus* a platform fee:

    platform_fee = round(plan_price * PLATFORM_FEE_PERCENT / 100)
    gst          = round(platform_fee * PLATFORM_FEE_GST_PERCENT / 100)   # INR only
    total        = plan_price + platform_fee + gst

All amounts are handled in minor units (paise for INR, cents for USD) so
there is no floating point drift. The client never supplies an amount -
every figure is recalculated server-side from the tier table.

Endpoints
---------
GET  /api/billing/config                 -> gateway status + fee config
GET  /api/billing/tiers                  -> public price list (USD + INR)
POST /api/billing/quote                  -> public order breakdown
POST /api/billing/create-order           -> auth, creates Razorpay order
POST /api/billing/verify-payment         -> auth, verifies signature + fulfils
POST /api/billing/webhook                -> Razorpay server-to-server webhook
GET  /api/billing/history                -> auth, customer payment history
PUT  /api/billing/tiers/<tier_id>        -> super_admin price update
"""
import hashlib
import hmac
import json
import os
import uuid
from datetime import datetime, timezone
from decimal import Decimal, ROUND_HALF_UP

from flask import Blueprint, current_app, jsonify, request
from sqlalchemy import inspect, text
from sqlalchemy.exc import IntegrityError

try:
    import razorpay
except ImportError:  # pragma: no cover - requirements.txt installs it
    razorpay = None

from .extensions import db, limiter
from .models import (
    AuditLog,
    Organization,
    OrganizationScanQuota,
    Payment,
    SubscriptionTier,
    User,
)
from .routes import token_required

billing_bp = Blueprint('billing', __name__)

SUPPORTED_CURRENCIES = ('INR', 'USD')
SCANS_PER_PURCHASE = 3

# Tier id -> scan_type used by OrganizationScanQuota
TIER_SCAN_TYPE = {
    'quick': 'Quick',
    'advanced': 'Advanced',
    'deep': 'Deep',
}

DEFAULT_TIERS = [
    {
        'id': 'free',
        'name': 'Free Tier Plan',
        'monthly_price': 0,
        'yearly_price': 0,
        'monthly_price_inr': 0,
        'yearly_price_inr': 0,
    },
    {
        'id': 'quick',
        'name': 'Quick Scan',
        'monthly_price': 499,        # $4.99
        'yearly_price': 4990,
        'monthly_price_inr': 49900,  # Rs.499
        'yearly_price_inr': 499000,
    },
    {
        'id': 'advanced',
        'name': 'Advanced Scan',
        'monthly_price': 4499,        # $44.99
        'yearly_price': 44990,
        'monthly_price_inr': 399900,  # Rs.3,999
        'yearly_price_inr': 3999000,
    },
    {
        'id': 'deep',
        'name': 'Deep Scan',
        'monthly_price': 9999,        # $99.99
        'yearly_price': 99990,
        'monthly_price_inr': 899900,  # Rs.8,999
        'yearly_price_inr': 8999000,
    },
    {
        'id': 'enterprise',
        'name': 'Custom Solutions',
        'monthly_price': 0,
        'yearly_price': 0,
        'monthly_price_inr': 0,
        'yearly_price_inr': 0,
    },
]

_TIER_ORDER = {'quick': 0, 'advanced': 1, 'deep': 2, 'enterprise': 3}


# ---------------------------------------------------------------------------
# Configuration helpers (read lazily so HF secrets / .env changes are picked up)
# ---------------------------------------------------------------------------
def _key_id() -> str:
    return (os.getenv('RAZORPAY_KEY_ID') or '').strip()


def _key_secret() -> str:
    return (os.getenv('RAZORPAY_KEY_SECRET') or '').strip()


def _webhook_secret() -> str:
    return (os.getenv('RAZORPAY_WEBHOOK_SECRET') or '').strip()


def _fee_percent() -> Decimal:
    try:
        return Decimal(str(os.getenv('PLATFORM_FEE_PERCENT', '2')))
    except Exception:
        return Decimal('2')


def _gst_percent() -> Decimal:
    try:
        return Decimal(str(os.getenv('PLATFORM_FEE_GST_PERCENT', '18')))
    except Exception:
        return Decimal('18')


def _usd_to_inr_rate() -> Decimal:
    try:
        return Decimal(str(os.getenv('USD_TO_INR_RATE', '88')))
    except Exception:
        return Decimal('88')


def _client():
    if razorpay is None:
        raise RuntimeError('The "razorpay" Python package is not installed.')
    if not _key_id() or not _key_secret():
        raise RuntimeError('RAZORPAY_KEY_ID / RAZORPAY_KEY_SECRET are not configured.')
    return razorpay.Client(auth=(_key_id(), _key_secret()))


def _minor_to_major(minor) -> str:
    return f"{(int(minor or 0) / 100):.2f}"


def _round_minor(value: Decimal) -> int:
    return int(value.quantize(Decimal('1'), rounding=ROUND_HALF_UP))


def _fee_breakdown(base_minor: int, currency: str) -> dict:
    """Return the platform-fee breakdown for a plan price, in minor units."""
    currency = currency.upper()
    base = Decimal(int(base_minor or 0))

    fee = _round_minor(base * _fee_percent() / Decimal('100'))
    # GST is charged on the platform fee for domestic (INR) invoices only.
    gst = _round_minor(Decimal(fee) * _gst_percent() / Decimal('100')) if currency == 'INR' else 0
    total = int(base) + fee + gst

    return {
        'currency': currency,
        'base_amount': int(base),
        'platform_fee': fee,
        'tax_amount': gst,
        'total_amount': total,
        'fee_percent': float(_fee_percent()),
        'gst_percent': float(_gst_percent()) if currency == 'INR' else 0.0,
        'base_display': _minor_to_major(base),
        'platform_fee_display': _minor_to_major(fee),
        'tax_display': _minor_to_major(gst),
        'total_display': _minor_to_major(total),
        'scans_included': SCANS_PER_PURCHASE,
    }


def _tier_price_minor(tier: SubscriptionTier, currency: str) -> int:
    """Plan price in minor units for the requested currency."""
    if currency == 'INR':
        if tier.monthly_price_inr and tier.monthly_price_inr > 0:
            return int(tier.monthly_price_inr)
        # Legacy row without an INR price: convert from USD cents using the
        # configured rate (1 USD cent -> `rate` paise).
        return _round_minor(Decimal(int(tier.monthly_price or 0)) * _usd_to_inr_rate())
    return int(tier.monthly_price or 0)


def _get_tier(tier_id: str):
    return db.session.get(SubscriptionTier, (tier_id or '').strip().lower())


def _payment_dict(payment: Payment) -> dict:
    return {
        'id': payment.id,
        'tier_id': payment.tier_id,
        'currency': payment.currency,
        'amount': payment.amount,
        'amount_display': _minor_to_major(payment.amount),
        'base_amount': payment.base_amount,
        'platform_fee': payment.platform_fee,
        'tax_amount': payment.tax_amount,
        'status': payment.status,
        'razorpay_order_id': payment.razorpay_order_id,
        'razorpay_payment_id': payment.razorpay_payment_id,
        'created_at': payment.created_at.isoformat() + 'Z' if payment.created_at else None,
    }


# ---------------------------------------------------------------------------
# Schema upgrade + seed (safe to call on every boot)
# ---------------------------------------------------------------------------
def ensure_billing_schema():
    """Add billing columns that were introduced after the first deployment.

    `db.create_all()` only creates *missing tables*, never missing columns,
    so existing Cloud SQL databases need this tiny in-place migration.
    Works on both PostgreSQL and SQLite.
    """
    inspector = inspect(db.engine)
    try:
        table_names = set(inspector.get_table_names())
    except Exception as exc:  # pragma: no cover - DB unreachable
        current_app.logger.warning(f"[Billing] Could not inspect schema: {exc}")
        return

    statements = []

    if 'subscription_tiers' in table_names:
        cols = {c['name'] for c in inspector.get_columns('subscription_tiers')}
        if 'monthly_price_inr' not in cols:
            statements.append("ALTER TABLE subscription_tiers ADD COLUMN monthly_price_inr INTEGER DEFAULT 0")
        if 'yearly_price_inr' not in cols:
            statements.append("ALTER TABLE subscription_tiers ADD COLUMN yearly_price_inr INTEGER DEFAULT 0")

    if 'payments' in table_names:
        cols = {c['name'] for c in inspector.get_columns('payments')}
        if 'base_amount' not in cols:
            statements.append("ALTER TABLE payments ADD COLUMN base_amount INTEGER")
        if 'platform_fee' not in cols:
            statements.append("ALTER TABLE payments ADD COLUMN platform_fee INTEGER")
        if 'tax_amount' not in cols:
            statements.append("ALTER TABLE payments ADD COLUMN tax_amount INTEGER")

    for stmt in statements:
        try:
            db.session.execute(text(stmt))
            db.session.commit()
            current_app.logger.info(f"[Billing] Applied migration: {stmt}")
        except Exception as exc:
            db.session.rollback()
            current_app.logger.warning(f"[Billing] Migration skipped ({stmt}): {exc}")


def seed_billing_tiers():
    """Create the four plans the pricing page sells, if they are missing."""
    changed = False
    for spec in DEFAULT_TIERS:
        tier = db.session.get(SubscriptionTier, spec['id'])
        if tier is None:
            db.session.add(SubscriptionTier(**spec))
            changed = True
            continue

        # Fill in newer columns for rows created by an older seed script.
        if not tier.monthly_price_inr and spec['monthly_price_inr']:
            tier.monthly_price_inr = spec['monthly_price_inr']
            changed = True
        if not tier.yearly_price_inr and spec['yearly_price_inr']:
            tier.yearly_price_inr = spec['yearly_price_inr']
            changed = True

    if changed:
        try:
            db.session.commit()
            current_app.logger.info("[Billing] Seeded subscription tiers.")
        except Exception as exc:
            db.session.rollback()
            current_app.logger.warning(f"[Billing] Tier seed failed: {exc}")


# ---------------------------------------------------------------------------
# Public endpoints
# ---------------------------------------------------------------------------
@billing_bp.route('/config', methods=['GET'])
def billing_config():
    return jsonify({
        'provider': 'razorpay',
        'configured': bool(_key_id() and _key_secret()),
        'webhook_configured': bool(_webhook_secret()),
        'currencies': list(SUPPORTED_CURRENCIES),
        'platform_fee_percent': float(_fee_percent()),
        'gst_percent': float(_gst_percent()),
        'gst_applies_to': ['INR'],
        'scans_per_purchase': SCANS_PER_PURCHASE,
    }), 200


@billing_bp.route('/tiers', methods=['GET'])
def list_tiers():
    tiers = [t for t in SubscriptionTier.query.all() if t.id in _TIER_ORDER]
    tiers.sort(key=lambda t: _TIER_ORDER.get(t.id, 99))

    payload = []
    for tier in tiers:
        payload.append({
            'id': tier.id,
            'name': tier.name,
            # Minor units. USD in cents, INR in paise.
            'monthly_price': int(tier.monthly_price or 0),
            'yearly_price': int(tier.yearly_price or 0),
            'monthly_price_inr': int(_tier_price_minor(tier, 'INR')),
            'yearly_price_inr': int(tier.yearly_price_inr or 0),
            'purchasable': tier.id in TIER_SCAN_TYPE,
            'scans_included': SCANS_PER_PURCHASE,
        })
    return jsonify(payload), 200


@billing_bp.route('/quote', methods=['POST'])
@limiter.limit("120 per minute")
def quote():
    data = request.get_json(silent=True) or {}
    tier_id = str(data.get('tier_id') or '').strip().lower()
    currency = str(data.get('currency') or 'INR').strip().upper()

    if currency not in SUPPORTED_CURRENCIES:
        return jsonify({'message': f'Unsupported currency. Use one of {SUPPORTED_CURRENCIES}.'}), 400

    tier = _get_tier(tier_id)
    if tier is None or tier.id not in TIER_SCAN_TYPE:
        return jsonify({'message': 'Unknown or non-purchasable plan.'}), 400

    base = _tier_price_minor(tier, currency)
    if base <= 0:
        return jsonify({'message': 'This plan is not available for online checkout.'}), 400

    breakdown = _fee_breakdown(base, currency)
    breakdown.update({
        'tier_id': tier.id,
        'tier_name': tier.name,
    })
    return jsonify(breakdown), 200


# ---------------------------------------------------------------------------
# Authenticated checkout
# ---------------------------------------------------------------------------
@billing_bp.route('/create-order', methods=['POST'])
@token_required
@limiter.limit("20 per minute")
def create_order(current_user):
    data = request.get_json(silent=True) or {}
    tier_id = str(data.get('tier_id') or data.get('price_id') or '').strip().lower()
    currency = str(data.get('currency') or 'INR').strip().upper()

    if currency not in SUPPORTED_CURRENCIES:
        return jsonify({'message': f'Unsupported currency. Use one of {SUPPORTED_CURRENCIES}.'}), 400

    tier = _get_tier(tier_id)
    if tier is None or tier.id not in TIER_SCAN_TYPE:
        return jsonify({'message': 'Unknown or non-purchasable plan.'}), 400

    if not current_user.org_id:
        return jsonify({'message': 'Your account is not linked to an organization. Please contact support.'}), 400

    base = _tier_price_minor(tier, currency)
    if base <= 0:
        return jsonify({'message': 'This plan is not available for online checkout.'}), 400

    breakdown = _fee_breakdown(base, currency)

    if breakdown['total_amount'] < 100:
        # Razorpay rejects amounts below 1.00 unit for most methods.
        return jsonify({'message': 'Order amount is too small for Razorpay.'}), 400

    try:
        client = _client()
    except RuntimeError as exc:
        current_app.logger.error(f"[Billing] Razorpay not configured: {exc}")
        return jsonify({'message': 'Payment gateway is not configured yet. Please contact support.'}), 503

    receipt = f"larshield_{uuid.uuid4().hex[:24]}"
    notes = {
        'user_id': current_user.id,
        'org_id': current_user.org_id,
        'tier_id': tier.id,
        'base_amount': str(breakdown['base_amount']),
        'platform_fee': str(breakdown['platform_fee']),
        'tax_amount': str(breakdown['tax_amount']),
        'total_amount': str(breakdown['total_amount']),
        'scans': str(SCANS_PER_PURCHASE),
    }

    try:
        order = client.order.create({
            'amount': int(breakdown['total_amount']),
            'currency': currency,
            'receipt': receipt,
            'notes': notes,
        })
    except Exception as exc:
        current_app.logger.error(f"[Billing] Razorpay order creation failed: {exc}")
        return jsonify({'message': 'Could not start the payment. Please try again in a moment.'}), 502

    payment = Payment(
        user_id=current_user.id,
        org_id=current_user.org_id,
        tier_id=tier.id,
        amount=int(breakdown['total_amount']),
        base_amount=int(breakdown['base_amount']),
        platform_fee=int(breakdown['platform_fee']),
        tax_amount=int(breakdown['tax_amount']),
        currency=currency,
        status='created',
        razorpay_order_id=order.get('id'),
    )
    db.session.add(payment)
    db.session.commit()

    return jsonify({
        'order_id': order.get('id'),
        'key_id': _key_id(),
        'amount': int(breakdown['total_amount']),
        'currency': currency,
        'breakdown': breakdown,
        'tier': {
            'id': tier.id,
            'name': tier.name,
            'scans_included': SCANS_PER_PURCHASE,
        },
        'description': f"{tier.name} - {SCANS_PER_PURCHASE} scans for 1 target website",
        'prefill': {
            'name': f"{current_user.first_name or ''} {current_user.last_name or ''}".strip(),
            'email': current_user.email,
        },
        'payment_id': payment.id,
    }), 200


def _fulfil(payment: Payment, razorpay_payment_id: str) -> Payment:
    """Mark a payment successful and grant the purchased scans (idempotent)."""
    if payment.status == 'successful' and payment.razorpay_payment_id:
        return payment

    user = db.session.get(User, payment.user_id)
    org = db.session.get(Organization, payment.org_id) if payment.org_id else None
    if org is None and user is not None:
        org = user.organization
    if org is None:
        raise ValueError('No organization linked to this payment.')

    payment.status = 'successful'
    payment.razorpay_payment_id = razorpay_payment_id
    if not payment.created_at:
        payment.created_at = datetime.now(timezone.utc)

    # Upgrade the tenant tier. Tier ids are seeded into subscription_tiers,
    # so the organizations.subscription_tier foreign key is satisfied.
    org.subscription_tier = payment.tier_id

    scan_type = TIER_SCAN_TYPE.get(payment.tier_id, payment.tier_id.capitalize())
    quota = OrganizationScanQuota.query.filter_by(org_id=org.id, scan_type=scan_type).first()
    if quota is None:
        db.session.add(OrganizationScanQuota(
            org_id=org.id,
            scan_type=scan_type,
            allocated_count=SCANS_PER_PURCHASE,
            used_count=0,
        ))
    else:
        quota.allocated_count = int(quota.allocated_count or 0) + SCANS_PER_PURCHASE

    db.session.add(AuditLog(
        admin_id=payment.user_id,
        action=(
            f"Razorpay payment successful: {payment.tier_id} plan "
            f"({payment.currency} {_minor_to_major(payment.amount)}) - "
            f"+{SCANS_PER_PURCHASE} {scan_type} scans"
        ),
        target_id=org.id,
    ))

    try:
        db.session.commit()
    except IntegrityError:
        # Another worker (webhook vs redirect verify) already fulfilled it.
        db.session.rollback()
        existing = Payment.query.filter_by(razorpay_payment_id=razorpay_payment_id).first()
        if existing is not None:
            return existing
        raise

    return payment


@billing_bp.route('/verify-payment', methods=['POST'])
@token_required
@limiter.limit("30 per minute")
def verify_payment(current_user):
    data = request.get_json(silent=True) or {}
    order_id = str(data.get('razorpay_order_id') or '').strip()
    payment_id = str(data.get('razorpay_payment_id') or '').strip()
    signature = str(data.get('razorpay_signature') or '').strip()

    if not order_id or not payment_id or not signature:
        return jsonify({'message': 'razorpay_order_id, razorpay_payment_id and razorpay_signature are required.'}), 400

    secret = _key_secret()
    if not secret:
        return jsonify({'message': 'Payment gateway is not configured yet.'}), 503

    expected = hmac.new(
        secret.encode('utf-8'),
        f"{order_id}|{payment_id}".encode('utf-8'),
        hashlib.sha256,
    ).hexdigest()
    if not hmac.compare_digest(expected, signature):
        current_app.logger.warning(f"[Billing] Signature mismatch for order {order_id}")
        return jsonify({'message': 'Invalid payment signature.'}), 400

    payment = Payment.query.filter_by(razorpay_order_id=order_id).first()
    if payment is None:
        return jsonify({'message': 'Payment record not found.'}), 404

    if payment.user_id != current_user.id and current_user.role not in ('super_admin', 'admin'):
        return jsonify({'message': 'This payment belongs to another account.'}), 403

    if payment.status == 'successful':
        return jsonify({'status': 'success', 'message': 'Payment already verified.', 'payment': _payment_dict(payment)}), 200

    # Server-side confirmation with Razorpay. The signature proves the payment
    # is genuine, but we also confirm the captured amount/currency/order.
    try:
        rp_payment = _client().payment.fetch(payment_id)
    except Exception as exc:
        current_app.logger.error(f"[Billing] Could not fetch payment {payment_id}: {exc}")
        return jsonify({'message': 'Could not confirm the payment with the gateway. Please contact support.'}), 502

    if str(rp_payment.get('order_id') or '') != order_id:
        return jsonify({'message': 'Payment does not belong to this order.'}), 400
    if str(rp_payment.get('status') or '').lower() not in ('captured', 'authorized'):
        return jsonify({'message': f"Payment is not completed (status: {rp_payment.get('status')})."}), 402
    if int(rp_payment.get('amount') or 0) != int(payment.amount):
        current_app.logger.error(
            f"[Billing] Amount mismatch order {order_id}: gateway={rp_payment.get('amount')} expected={payment.amount}"
        )
        return jsonify({'message': 'Paid amount does not match the order.'}), 400
    if str(rp_payment.get('currency') or '').upper() != str(payment.currency).upper():
        return jsonify({'message': 'Paid currency does not match the order.'}), 400

    try:
        payment = _fulfil(payment, payment_id)
    except ValueError as exc:
        db.session.rollback()
        return jsonify({'message': str(exc)}), 400
    except Exception as exc:
        db.session.rollback()
        current_app.logger.exception(f"[Billing] Fulfilment failed for order {order_id}: {exc}")
        return jsonify({'message': 'Payment succeeded but activation failed. Please contact support with your order id.'}), 500

    return jsonify({
        'status': 'success',
        'message': 'Payment verified and plan activated.',
        'payment': _payment_dict(payment),
    }), 200


# ---------------------------------------------------------------------------
# Webhook (source of truth if the user closes the tab before redirect)
# ---------------------------------------------------------------------------
@billing_bp.route('/webhook', methods=['POST'])
@limiter.exempt
def razorpay_webhook():
    secret = _webhook_secret()
    if not secret:
        current_app.logger.warning("[Billing] Webhook received but RAZORPAY_WEBHOOK_SECRET is not set.")
        return jsonify({'status': 'ignored'}), 200

    raw_body = request.get_data() or b''
    signature = request.headers.get('X-Razorpay-Signature', '')
    expected = hmac.new(secret.encode('utf-8'), raw_body, hashlib.sha256).hexdigest()

    if not hmac.compare_digest(expected, signature):
        current_app.logger.warning("[Billing] Webhook signature verification failed.")
        return jsonify({'status': 'invalid signature'}), 401

    try:
        payload = json.loads(raw_body.decode('utf-8') or '{}')
    except (json.JSONDecodeError, UnicodeDecodeError):
        return jsonify({'status': 'invalid payload'}), 400

    event = payload.get('event') or ''
    body = payload.get('payload') or {}
    payment_entity = (body.get('payment') or {}).get('entity') or {}
    order_entity = (body.get('order') or {}).get('entity') or {}

    order_id = payment_entity.get('order_id') or order_entity.get('id')
    payment_id = payment_entity.get('id')

    if event in ('payment.captured', 'order.paid') and order_id:
        row = Payment.query.filter_by(razorpay_order_id=order_id).first()
        if row is None:
            current_app.logger.warning(f"[Billing] Webhook for unknown order {order_id}")
            return jsonify({'status': 'ok'}), 200

        if row.status == 'successful':
            return jsonify({'status': 'ok'}), 200

        if payment_entity.get('amount') and int(payment_entity['amount']) != int(row.amount):
            current_app.logger.error(
                f"[Billing] Webhook amount mismatch for {order_id}: "
                f"gateway={payment_entity.get('amount')} expected={row.amount}"
            )
            return jsonify({'status': 'amount mismatch'}), 200

        if not payment_id:
            current_app.logger.warning(f"[Billing] Webhook {event} for {order_id} has no payment id yet; skipping.")
            return jsonify({'status': 'ok'}), 200

        try:
            _fulfil(row, payment_id)
        except Exception as exc:
            db.session.rollback()
            current_app.logger.exception(f"[Billing] Webhook fulfilment failed for {order_id}: {exc}")
            return jsonify({'status': 'retry'}), 500

    elif event == 'payment.failed' and order_id:
        row = Payment.query.filter_by(razorpay_order_id=order_id).first()
        if row is not None and row.status == 'created':
            row.status = 'failed'
            db.session.commit()
            current_app.logger.info(f"[Billing] Payment failed for order {order_id}")

    return jsonify({'status': 'ok'}), 200


# ---------------------------------------------------------------------------
# Customer payment history (Settings -> Billing)
# ---------------------------------------------------------------------------
@billing_bp.route('/history', methods=['GET'])
@token_required
def billing_history(current_user):
    if not current_user.org_id:
        return jsonify({'history': []}), 200

    payments = (
        Payment.query
        .filter_by(org_id=current_user.org_id)
        .order_by(Payment.created_at.desc())
        .limit(100)
        .all()
    )

    history = []
    for p in payments:
        history.append({
            'id': p.id,
            'tier_id': p.tier_id,
            # Minor units - the frontend divides by 100 for display.
            'amount': int(p.amount or 0),
            'base_amount': p.base_amount,
            'platform_fee': p.platform_fee,
            'tax_amount': p.tax_amount,
            'currency': p.currency or 'USD',
            # Historical UI expects "success" rather than "successful".
            'status': 'success' if p.status == 'successful' else p.status,
            'created_at': p.created_at.isoformat() + 'Z' if p.created_at else None,
        })
    return jsonify({'history': history}), 200


# ---------------------------------------------------------------------------
# Admin
# ---------------------------------------------------------------------------
@billing_bp.route('/tiers/<tier_id>', methods=['PUT'])
@token_required
def update_tier(current_user, tier_id):
    if current_user.role != 'super_admin':
        return jsonify({'message': 'Admin privileges required!'}), 403

    tier = _get_tier(tier_id)
    if tier is None:
        return jsonify({'message': 'Tier not found.'}), 404

    data = request.get_json(silent=True) or {}
    fields = ('monthly_price', 'yearly_price', 'monthly_price_inr', 'yearly_price_inr')
    updated = []

    for field in fields:
        if field in data:
            try:
                value = int(data[field])
            except (TypeError, ValueError):
                return jsonify({'message': f'{field} must be an integer (minor units).'}), 400
            if value < 0:
                return jsonify({'message': f'{field} cannot be negative.'}), 400
            setattr(tier, field, value)
            updated.append(field)

    if updated:
        db.session.add(AuditLog(
            admin_id=current_user.id,
            action=f"Updated billing tier {tier.id}: {', '.join(updated)}",
            target_id=tier.id,
        ))
        db.session.commit()

    return jsonify({
        'message': 'Tier updated successfully.',
        'tier': {
            'id': tier.id,
            'name': tier.name,
            'monthly_price': tier.monthly_price,
            'yearly_price': tier.yearly_price,
            'monthly_price_inr': tier.monthly_price_inr,
            'yearly_price_inr': tier.yearly_price_inr,
        },
    }), 200