feat(quality): add image quality gate before forensic analysis

backend/utils/image_quality.py — assess_image_quality() checks images
before expensive processing:
- Rejects images < 64x64 with HTTP 422
- Warns on images < 256x256 (reduced confidence_cap to 0.70)
- Warns on grayscale, RGBA, heavily-compressed JPEG
- Returns confidence_cap to cap signal confidence appropriately

analyze.py: quality gate added as Layer 4 between MIME validation and
cache lookup. Prevents wasted computation and inaccurate analysis on
solid-color, tiny, or corrupt images.

backend/utils/image_quality.py

@@ -0,0 +1,85 @@
+"""
+Image Quality Gate for VeriFile-X.
+
+Checks image quality before running expensive forensic analysis.
+Rejects or flags images that are too small, corrupt, or unreadable.
+
+Quality tiers:
+  good       - Full analysis, all signals active
+  degraded   - Analysis runs but some signals unreliable
+  low        - Analysis runs with reduced confidence
+  unsuitable - Reject (too small, corrupt)
+"""
+import logging
+from typing import Dict, Any
+from io import BytesIO
+
+logger = logging.getLogger(__name__)
+
+MIN_WIDTH  = 64
+MIN_HEIGHT = 64
+
+RECOMMENDED_MIN_WIDTH  = 256
+RECOMMENDED_MIN_HEIGHT = 256
+
+
+def assess_image_quality(image_bytes: bytes, filename: str = "unknown") -> Dict[str, Any]:
+    from PIL import Image
+    warnings = []
+
+    try:
+        img          = Image.open(BytesIO(image_bytes))
+        width, height = img.size
+        fmt          = img.format or "unknown"
+        mode         = img.mode
+        pixels       = width * height
+    except Exception:
+        logger.warning("Image quality check failed for %s", filename)
+        return {
+            "tier": "unsuitable", "suitable": False,
+            "width": 0, "height": 0, "pixel_count": 0,
+            "format": "unknown", "mode": "unknown",
+            "warnings": ["Image could not be decoded"],
+            "confidence_cap": 0.0,
+            "reason": "Image is corrupt or unreadable",
+        }
+
+    if width < MIN_WIDTH or height < MIN_HEIGHT:
+        return {
+            "tier": "unsuitable", "suitable": False,
+            "width": width, "height": height, "pixel_count": pixels,
+            "format": fmt, "mode": mode,
+            "warnings": [f"Image too small ({width}x{height}). Minimum: {MIN_WIDTH}x{MIN_HEIGHT}"],
+            "confidence_cap": 0.0,
+            "reason": f"Image {width}x{height} is below minimum {MIN_WIDTH}x{MIN_HEIGHT} for forensic analysis.",
+        }
+
+    if width < RECOMMENDED_MIN_WIDTH or height < RECOMMENDED_MIN_HEIGHT:
+        warnings.append(f"Image is small ({width}x{height}). Less reliable below {RECOMMENDED_MIN_WIDTH}x{RECOMMENDED_MIN_HEIGHT}.")
+
+    if mode in ("1", "P"):
+        warnings.append(f"Color mode {mode} has limited forensic information.")
+    if mode == "L":
+        warnings.append("Grayscale image — color-based signals will be inactive.")
+    if mode == "RGBA":
+        warnings.append("RGBA image — alpha channel ignored.")
+
+    bytes_per_pixel = len(image_bytes) / max(pixels, 1)
+    if fmt == "JPEG" and bytes_per_pixel < 0.3:
+        warnings.append(f"Heavily compressed JPEG ({bytes_per_pixel:.2f} bytes/pixel) may interfere with some signals.")
+
+    if width < RECOMMENDED_MIN_WIDTH or height < RECOMMENDED_MIN_HEIGHT:
+        tier, confidence_cap = "low", 0.70
+    elif warnings:
+        tier, confidence_cap = "degraded", 0.85
+    else:
+        tier, confidence_cap = "good", 1.0
+
+    logger.info("Image quality %s: tier=%s (%dx%d %s %s)", filename, tier, width, height, fmt, mode)
+
+    return {
+        "tier": tier, "suitable": True,
+        "width": width, "height": height, "pixel_count": pixels,
+        "format": fmt, "mode": mode,
+        "warnings": warnings, "confidence_cap": confidence_cap, "reason": None,
+    }