fix(api): C-04 -- /investigate was using DeepInvestigator not MultiInvestigator

The /investigate/{id} endpoint used DeepInvestigator (6-layer graph
traversal) but the homepage claimed '15 specialist AI investigators
run in parallel'. MultiInvestigator -- the one that actually runs all
15 specialists (financial, political, corporate, judicial, procurement,
network, asset, international, media, historical, public_interest,
doubt, affidavit, benami, math) -- was only instantiated inside the
PDF export route and never triggered by normal usage.

Fix: /investigate now uses MultiInvestigator by default.
Added mode=deep param for backward compat with existing integrations.

api/routes/investigation.py

@@ -7,6 +7,7 @@ from loguru import logger
 
 from api.dependencies import get_db
 from ai.deep_investigator import DeepInvestigator
+from ai.multi_investigator import MultiInvestigator
 from ai.connection_mapper import ConnectionMapper
 
 router = APIRouter()