feat: playlist shuffle, lazy resolution, TTS ducking, kept repair

Music:
- #random URL fragment shuffles playlist tracks before enqueuing
- Lazy playlist resolution: first 10 tracks resolve immediately,
  remaining are fetched in a background task
- !kept repair re-downloads kept tracks with missing local files
- !kept shows [MISSING] marker for tracks without local files
- TTS ducking: music ducks when merlin speaks via voice peer,
  smooth restore after TTS finishes

Performance (from profiling):
- Connection pool: preload_content=True for SOCKS connection reuse
- Pool tuning: 30 pools / 8 connections (up from 20/4)
- _PooledResponse wrapper for stdlib-compatible read interface
- Iterative _extract_videos (replace 51K-deep recursion with stack)
- proxy=False for local SearXNG

Voice + multi-bot:
- Per-bot voice config lookup ([<username>.voice] in TOML)
- Mute detection: skip duck silence when all users muted
- Autoplay shuffle deck (no repeats until full cycle)
- Seek clamp to track duration (prevent seek-past-end stall)

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

docs/USAGE.md

@@ -1628,7 +1628,7 @@ and voice transmission.
 !np                  Now playing
 !volume [0-100]      Get/set volume (persisted across restarts)
 !keep                Keep current track's audio file (with metadata)
-!kept [clear]        List kept files with metadata, or clear all
+!kept [clear|repair] List kept files, clear all, or re-download missing
 !testtone            Play 3-second 440Hz test tone
 ```
 
@@ -1751,6 +1751,8 @@ file (natural dedup).
 - Use `!kept` to list preserved files with metadata (title, artist, duration,
   file size)
 - Use `!kept clear` to delete all preserved files and their metadata
+- Use `!kept repair` to re-download any kept tracks whose local files are
+  missing (e.g. after a cleanup or volume mount issue)
 - On cancel/error, files are not deleted (needed for `!resume`)
 
 ### Extra Mumble Bots