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

docker-compose.yml

@@ -4,8 +4,23 @@ services:
     restart: unless-stopped
     env_file:
       - .env
+    environment:
+      # Health check port (exposed to host for docker healthcheck)
+      - HEALTH_PORT=8765
+      # Scheduler staleness threshold in seconds
+      - HEALTHCHECK_SCHEDULER_STALE_S=300
     networks:
       - supabase_default
+    # Graceful shutdown timeout — Docker sends SIGTERM, container has this long to clean up.
+    stop_grace_period: 15s
+    # Docker healthcheck (uses the in-container aiohttp server)
+    healthcheck:
+      test: ["CMD", "python", "-c",
+             "import httpx,os; r=httpx.get(f'http://localhost:{os.getenv(\"HEALTH_PORT\",\"8765\")}/health'); r.raise_for_status()"]
+      interval: 15s
+      timeout: 5s
+      retries: 3
+      start_period: 10s
 
 networks:
   supabase_default: