Server-side request forgery in Blombooru image proxy

Blombooru is a self-hosted personal booru for organizing and tagging private image collections. Prior to 1.40.0, the administrator-only `GET /proxy-image` endpoint, exposed as `/api/booru-import/proxy-image`, accepts a `url` query parameter and fetches it server-side without meaningful destination validation. `proxy_image()` in `backend/app/routes/booru_import.py` only checks whether the value starts with http, then uses `get_client_for_url` or `requests.get` and streams the response. Private, loopback and link-local addresses are not blocked, and automatically followed redirects can move a request from a trusted domain to an internal address. An authenticated administrator can enumerate internal services and access cloud instance metadata, potentially disclosing IAM credentials and crossing the application's intended cloud or network trust boundary. The reporter also observed a hanging recursive self-request. Administrator authentication remains a prerequisite; this is not an anonymous proxy. This issue is fixed in version 1.40.0.