Implement Phase 0b base project against the frozen test contract

All seven services (gateway, ledger, fraud, sanctions, insights, notifier,
audit), db/schema.sql plus optional_indexes.sql, scripts/seed.py and
smoke.py, the gateway-served frontend, and docker-compose.yml. Plain-text
stdlib logging and a weak manual X-Request-Id only -- no observability yet,
by design, per test_no_observability.py. Implements all 13 named chaos-flag
pathologies from docs/PATHOLOGIES.md behind Redis-backed runtime flags.

make smoke and make test-contract pass; make test-baseline and
make test-pathology pass except for a handful of load-sensitive tests
documented in tests/README.md rather than papered over.

requirements.txt

@@ -0,0 +1,10 @@
+fastapi==0.115.6
+uvicorn[standard]==0.34.0
+httpx==0.28.1
+sqlalchemy[asyncio]==2.0.36
+asyncpg==0.30.0
+psycopg[binary]==3.2.3
+redis==5.2.1
+pydantic==2.10.4
+pydantic-settings==2.7.1
+python-multipart==0.0.20