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.

common/db.py

@@ -0,0 +1,15 @@
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+
+from .settings import settings
+
+
+def make_engine(**overrides):
+    """A fresh async engine. Services that need more than one pool profile
+    (see ledger/chaos.py for pool_starvation) call this more than once."""
+    kwargs = dict(pool_pre_ping=True)
+    kwargs.update(overrides)
+    return create_async_engine(settings.postgres_dsn, **kwargs)
+
+
+engine = make_engine(pool_size=20, max_overflow=10)
+Session = async_sessionmaker(engine, expire_on_commit=False)