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.

db/optional_indexes.sql

@@ -0,0 +1,5 @@
+-- Applied only by `make fix-index` (docs/phase-0-base-project-spec.md §5, §12).
+-- These are the indexes deliberately omitted from db/schema.sql.
+
+CREATE INDEX IF NOT EXISTS transactions_account_created_idx ON transactions (account_id, created_at);
+CREATE INDEX IF NOT EXISTS transactions_category_idx ON transactions (category_id);