feat(phase-0): stability hardening — search path fix, auto-migrations, graceful shutdown, health endpoint

- Remove SET search_path from db.py and migration SQL (Supabase uses public schema)
- Add migrate.py with tracking table for forward-only SQL migrations
- Add entrypoint.sh: waits for DB, runs migrations, then starts app
- Copy 01-schema.sql + zz-seed.sql to worker/src/migrations/
- Add health.py: /health endpoint (200/503) with DB connectivity + scheduler staleness checks
  - /stats endpoint with keyword/ad/notification counts
- Rewrite main.py shutdown sequence: signal handler, 5s grace for scheduler, ordered cleanup
- Update Dockerfile: HEALTHCHECK directive, entrypoint, COPY migrations
- Update docker-compose.yml: stop_grace_period=15s, healthcheck config, env vars
- Add aiohttp>=3.9 to requirements.txt for health server

worker/src/db.py

@@ -7,9 +7,6 @@ logger = logging.getLogger(__name__)
 _pool: asyncpg.Pool | None = None
 
 
-async def _init_connection(conn: asyncpg.Connection) -> None:
-    await conn.execute("SET search_path TO willhaben_tracker")
-
 
 async def get_pool() -> asyncpg.Pool:
     global _pool
@@ -22,7 +19,6 @@ async def get_pool() -> asyncpg.Pool:
             database=os.getenv("POSTGRES_DB", "postgres"),
             min_size=2,
             max_size=10,
-            init=_init_connection
         )
         logger.info("Database pool initialized")
     return _pool