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/Dockerfile

@@ -1,6 +1,20 @@
 FROM python:3.12-slim
 WORKDIR /app
+
+# ── Dependencies ────────────────────────────────
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
+
+# ── Application code + migrations ───────────────
 COPY src/ .
+
+# Make entrypoint executable
+RUN chmod +x entrypoint.sh
+
+# ── Healthcheck (Docker-native) ─────────────────
+HEALTHCHECK --interval=15s --timeout=3s --retries=3 \
+    CMD python -c "import httpx,os; r=httpx.get(f'http://localhost:{os.getenv(\"HEALTH_PORT\",\"8765\")}/health'); r.raise_for_status()" || exit 1
+
+# ── Entrypoint: migrate then run ────────────────
+ENTRYPOINT ["/app/entrypoint.sh"]
 CMD ["python", "main.py"]