Internal Canary Feed

## Summary Koel's Subsonic `createPodcastChannel.view` endpoint accepts a user supplied podcast feed URL and fetches it server-side before applying the safe URL checks that are used for podcast episode enclosure URLs. An authenticated Subsonic API user can provide a loopback or internal URL as the feed URL and cause the Koel backend to issue a request to that address. A related redirect gap exists in the podcast stream helper: `PodcastService::getStreamableUrl()` validates only the original URL, then lets Guzzle follow redirects and accepts the final redirected URL without re-validating it. ## Impact An attacker with any valid Koel account and Subsonic API key can trigger server-side requests from the Koel host to loopback or internal network services. This can be used for blind SSRF against internal HTTP endpoints reachable by the Koel deployment. If an internal service returns valid RSS/XML or permissive CORS responses, parts of the response or final URL may be reflected back through normal podcast or stream behavior. ## Reproduction 1. Start Koel v9.6.0 or current main and create a normal user. 2. Obtain the user's Subsonic API key. 3. Start a local canary HTTP server on the Koel host at `127.0.0.1:8103` that records requests and returns this minimal RSS feed: ```xml Internal Canary Feed https://example.com/ Internal feed SSRF canary Episode One koel-internal-canary-episode-1 Mon, 01 Jun 2026 12:00:00 GMT ``` 4. Send an authenticated Subsonic request: ```http GET /rest/createPodcastChannel.view?apiKey=&f=json&url=http://127.0.0.1:8103/feed.xml HTTP/1.1 Host: koel.example ``` 5. The endpoint returns a successf