chore: add default value for FILES_URL

Signed-off-by: -LAN- <laipz8200@outlook.com>

docker/.env.example

 #   Recommendation: use a dedicated domain (e.g., https://upload.example.com).
 #   Alternatively, use http://<your-ip>:5001 or http://api:5001,
 #   ensuring port 5001 is externally accessible (see docker-compose.yaml).
-FILES_URL=
+FILES_URL=http://api:5001
 
 # INTERNAL_FILES_URL is used for plugin daemon communication within Docker network.
 # Set this to the internal Docker service URL for proper plugin file access.

docker/docker-compose.yaml

   SERVICE_API_URL: ${SERVICE_API_URL:-}
   APP_API_URL: ${APP_API_URL:-}
   APP_WEB_URL: ${APP_WEB_URL:-}
-  FILES_URL: ${FILES_URL:-}
+  FILES_URL: ${FILES_URL:-http://api:5001}
   INTERNAL_FILES_URL: ${INTERNAL_FILES_URL:-}
   LANG: ${LANG:-en_US.UTF-8}
   LC_ALL: ${LC_ALL:-en_US.UTF-8}