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/logging_config.py

@@ -0,0 +1,12 @@
+import logging
+
+
+def configure_logging(service_name: str) -> logging.Logger:
+    """Plain-text, human-prose logging -- the deliberately weak Phase 0
+    baseline. Per phase-0-base-project-spec.md §9: no JSON, no key-value, no
+    structured fields. Do not "improve" this; later phases replace it."""
+    logging.basicConfig(
+        level=logging.INFO,
+        format="%(asctime)s %(levelname)s %(name)s %(message)s",
+    )
+    return logging.getLogger(service_name)