Server-side request forgery in PinePods via podcast enclosure URLs

PinePods is a complete, self-hosted podcast management system written in Rust. Prior to 0.9.0, an authenticated user can subscribe to an attacker-controlled RSS feed through `POST /api/data/add_podcast` in `rust-api/src/handlers/podcasts.rs`. `get_podcast_values` and `add_episodes` in `rust-api/src/database.rs` parse the feed with `feed-rs` and store the `<enclosure url>` verbatim in `Episodes.episodeurl` without destination host/IP validation. `POST /api/data/download_podcast` invokes `spawn_download_podcast_episode` and `download_episode_and_wait()` in `rust-api/src/services/tasks.rs`, which sends a `reqwest` GET to the stored URL and saves and records the response in `DownloadedEpisodes` for later retrieval. The `estimate_duration_from_audio_url_async()` path in `rust-api/src/database.rs` also sends an unvalidated HEAD during feed add/refresh. Default redirect following permits an initially external enclosure URL to redirect to an internal target. A low-privilege user can make server-side GET/HEAD requests to reachable loopback, private-network and cloud metadata services and read downloaded internal responses, enabling internal-service enumeration, confidential-data disclosure and possible metadata credential theft. Episode downloading requires server downloads to be enabled, which is the default; self-service signup is supported. This issue is fixed in version 0.9.0.