docs: add music discovery, autoplay, and Mumble admin sections

- USAGE.md: music discovery (!similar, !tags), autoplay discovery
  config, Mumble admin (!mu) command reference
- CHEATSHEET.md: music discovery and Mumble admin quick reference
- ROADMAP.md: mark v2.4.0 as done, add MB fallback + !mu + autoplay
- TODO.md: mark music discovery and performance items as done
- PROJECT.md: update plugin categories table
- TASKS.md: close open doc items

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TODO.md

@@ -132,9 +132,9 @@ is preserved in git history for reference.
 
 ## Performance
 
-- [ ] Iterative `_extract_videos` in alert.py (51K recursive calls, 6.7s CPU)
-- [ ] Bypass SOCKS5 for local services (FlaskPaste, SearXNG)
-- [ ] Connection pool tuning (529 SOCKS connections per 25min session)
+- [x] Iterative `_extract_videos` in alert.py (replaced 51K-deep recursion)
+- [x] Bypass SOCKS5 for local services (SearXNG `proxy=False`)
+- [x] Connection pool tuning (30 pools / 8 connections)
 - [ ] Async HTTP client (aiohttp + aiohttp-socks) to avoid blocking executors
 - [x] Connection pooling via urllib3 SOCKSProxyManager
 - [x] Batch OG fetch via ThreadPoolExecutor
@@ -166,11 +166,11 @@ is preserved in git history for reference.
 
 ## Music Discovery
 
-- [ ] Last.fm integration (API key, free tier)
-- [ ] `!similar` command -- find similar artists/tracks via Last.fm
-- [ ] `!tags` command -- show genre/style tags for current track
-- [ ] Auto-queue similar tracks when autoplay has no kept tracks
-- [ ] MusicBrainz fallback (no API key, 1 req/sec rate limit)
+- [x] Last.fm integration (API key, free tier)
+- [x] `!similar` command -- find similar artists/tracks via Last.fm
+- [x] `!tags` command -- show genre/style tags for current track
+- [x] Auto-discover similar tracks during autoplay via Last.fm/MusicBrainz
+- [x] MusicBrainz fallback for `!similar` and `!tags` (no API key required)
 
 ## Slack