Bridge authentication hardened for forensic endpoints & Admin and internal surface lock-down

Release digest

Bridge authentication hardened for forensic endpoints
- Added signed request support (X-Key-Id, X-Timestamp, X-Nonce, X-Signature)
- Implemented HMAC-SHA256 canonical payload verification
- Added nonce replay protection (Redis-backed with in-memory fallback)
Admin and internal surface lock-down
- Admin router now enforces token auth consistently via router-level dependency
- Analytics, crypto, and corporate routes now support INTERNAL_API_KEY gate via X-Internal-Key
Error leakage sanitization completed on API/router surfaces
- Replaced external detail=str(e) patterns with sanitized error messages
- Added global safe error middleware with correlation error_id
Method mutation guard added
- Middleware blocks PUT/PATCH/DELETE outside allowed prefixes (default /api/admin)

Database pooling enabled
- Added shared ThreadedConnectionPool with safe proxy return-on-close behavior
- Added shutdown cleanup hook for pool close
Rate limiting centralized
- Added shared limiter key strategy (API key/internal key/IP)
- Applied forensic and crypto route limits with configurable env overrides
BigQuery cost controls enforced
- Added maximum_bytes_billed caps for standard and heavy forensic queries
- Added normalized forensic error codes mapped to stable HTTP responses
Load gate passed (50 concurrent users)
- Added scripts/perf/load_test_50_concurrent.py
- Generated LOAD_TEST_REPORT.md and artifacts/load_test_50_concurrent.json
- Baseline run passed all configured thresholds
Documentation synchronized
- Updated SECURITY.md, QUICKSTART.md, and README.md to match current hardening architecture and pre-release checklist