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.

services/notifier/app.py

@@ -0,0 +1,49 @@
+from contextlib import asynccontextmanager
+
+import asyncio
+
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
+
+from common.db import engine
+from common.logging_config import configure_logging
+from common.redis_client import redis_client
+
+from . import consumer
+
+SERVICE_NAME = "notifier"
+logger = configure_logging(SERVICE_NAME)
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    task = asyncio.create_task(consumer.run())
+    yield
+    task.cancel()
+
+
+app = FastAPI(title=SERVICE_NAME, lifespan=lifespan)
+
+
+@app.get("/")
+async def root():
+    return {"service": SERVICE_NAME, "version": "0.1.0"}
+
+
+@app.get("/healthz")
+async def healthz(request: Request):
+    if request.headers.get("X-Ledgerline-Probe") == "chatty-health":
+        logger.info("received chatty-health healthz probe from gateway")
+    return {"status": "ok", "service": SERVICE_NAME}
+
+
+@app.get("/readyz")
+async def readyz():
+    try:
+        async with engine.connect() as conn:
+            await conn.exec_driver_sql("SELECT 1")
+        await redis_client.ping()
+    except Exception as exc:
+        logger.exception(f"readyz check failed: {exc}")
+        return JSONResponse(status_code=503, content={"status": "unavailable"})
+    return {"status": "ok"}