Migrate from local Supabase stack to external Supabase instance

- Reduce docker-compose from 6 services to 1 (worker only)
- Connect via supavisor pooled port (6543) on dedicated Supabase
- Add willhaben_tracker schema with search_path in asyncpg pool init
- Create consolidated migration SQL for Supabase SQL Editor
- Update .env.example with new connection pattern

worker/src/db.py

@@ -18,6 +18,7 @@ async def get_pool() -> asyncpg.Pool:
             database=os.getenv("POSTGRES_DB", "postgres"),
             min_size=2,
             max_size=10,
+            init="SET search_path TO willhaben_tracker"
         )
         logger.info("Database pool initialized")
     return _pool