docs: plan Discover Mixes (auto-download, temporary) in backlog

Themed/random discovery mixes (Rock/Charts/Random) that auto-download via the
yt-dlp backend, auto-delete after a configurable retention time, with an option
to keep tracks in the permanent library. Captures sources, retention, keep flow,
and open questions for later implementation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -302,6 +302,12 @@ Primary causes identified:
 ## Planned / Backlog
 
 ### Features
+- **Web + Android** — **Discover Mixes (auto-download, temporary)** — themed/random mixes for discovering unknown music, Spotify-style (e.g. "Rock", "Charts", "Random Discovery"). Idea:
+  - Generate a mix from a theme/genre/charts (sources: Last.fm / ListenBrainz similar + iTunes charts), then **auto-download** the tracks via the yt-dlp backend so they play locally.
+  - **Temporary by default:** tracks **auto-delete after a configurable time** (e.g. 7 days) so the library doesn't fill up with stuff you didn't keep.
+  - **Keep what you like:** per-track (or whole-mix) option to **promote into the permanent library** instead of letting it expire.
+  - Goal: completely random/unfamiliar music you don't already know — like Spotify's curated/discovery playlists, but self-hosted.
+  - Open questions: where downloaded-but-temporary tracks live vs. the main library (separate folder + retention job), how Navidrome should/shouldn't index them, and dedupe against tracks you already own.
 - **Android** — Playlist drag-and-drop / sort order
 - **Android** — Artist page iTunes fallback (for artists not in Navidrome)
 - **Android** — Romaji / Japanese sort support