fix: nginx proxy buffering warnings and LOG_LEVEL propagation (#229)

- Increase proxy buffer sizes to prevent upstream image responses being
  buffered to temporary files (16k header + 8x256k body + 512k busy)
- Add env_file to frontend service in docker-compose.dev.yml for LOG_LEVEL
- Normalize LOG_LEVEL in nginx-entrypoint.sh (case-insensitive, trim whitespace)
- Add startup logging showing LOG_LEVEL → access_log mapping

Closes #226

docker-compose.dev.yml

@@ -30,6 +30,8 @@ services:
     volumes:
       - ./frontend:/app
       - frontend_node_modules:/app/node_modules
+    env_file:
+      - .env
     environment:
       - BACKEND_URL=http://backend-dev:3000
     ports: