Fix modular mode deployment issues (#1045)

- Fix Postgres version check failing with restricted DB users (use $POSTGRES_DB instead of hardcoded 'postgres')
- Fix DVR recording broken in modular mode (respect DISPATCHARR_PORT instead of hardcoding 9191)
- Remove flushdb() from wait_for_redis.py to prevent Redis data loss on container restart
- Add DISPATCHARR_PORT to celery environment in docker-compose.yml
- Add depends_on health conditions for proper service startup ordering
- Add extra_hosts for host.docker.internal resolution on Linux
- Harden celery entrypoint with timeouts for JWT wait (120s) and migration wait (300s)
- Replace fragile showmigrations grep with migrate --check
- Add unit tests for DVR port resolution and flushdb removal regression

scripts/wait_for_redis.py

@@ -31,7 +31,6 @@ def wait_for_redis(host='localhost', port=6379, db=0, password='', username='',
                 socket_connect_timeout=2
             )
             redis_client.ping()
-            redis_client.flushdb()  # Flush the database to ensure it's clean
             logger.info(f"✅ Redis at {host}:{port}/{db} is now available!")
             return True
         except (redis.exceptions.ConnectionError, redis.exceptions.TimeoutError) as e: