Subsonic API: any authenticated user can delete or read any other user's playlist (IDOR)

Summary

gonic is a music streaming server / free-software subsonic server API implementation. Prior to version 0.21.0, the Subsonic API endpoints `/rest/deletePlaylist.view` and `/rest/getPlaylist.view` perform no per-resource authorization. Once authenticated as any user (admin or not), an attacker can delete any playlist owned by any other user (including admin) by passing its `id` and read the full contents (name, comment, song list) of any other user's **private** (non-public) playlist by passing its `id`. The Subsonic playlist `id` is `base64url("<userID>/<filename>.m3u")`. Because filenames are user-supplied or time-derived and the `userID` is a small integer, IDs are guessable and frequently exposed (e.g. a previously-public playlist that was later made private still has the same ID). This breaks the multi-user trust boundary of gonic: a low-privileged user can wipe an administrator's curated playlists, and a user can exfiltrate any private playlist they obtain an ID for. The issue was fixed in commit `6dd71e6a3c966867ef8c900d359a7df75789f410`, which is part of version 0.21.0.