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

supabase/migrations/supabase-migration.sql

@@ -1,5 +1,5 @@
-CREATE SCHEMA IF NOT EXISTS willhaben_tracker;
-SET search_path TO willhaben_tracker;
+-- supabase-migration.sql — DEPRECATED: use 01-schema.sql instead (in migrations/)
+-- This file is kept for backward compatibility reference only.
 
 -- -----------------------------------------------------------
 -- users (whitelisted Telegram users)