feat: download audio before playback, add !keep and !kept commands

Audio is now downloaded to data/music/ before playback begins,
eliminating CDN hiccups mid-stream. Falls back to streaming on
download failure. Files are deleted after playback unless marked
with !keep. stream_audio detects local files and uses a direct
ffmpeg pipeline (no yt-dlp).

docs/USAGE.md

@@ -1574,6 +1574,8 @@ and voice transmission.
 !queue <url>         Add to queue (alias for !play)
 !np                  Now playing
 !volume [0-100]      Get/set volume
+!keep                Keep current track's audio file after playback
+!kept [clear]        List kept files or clear all
 !testtone            Play 3-second 440Hz test tone
 ```
 
@@ -1585,7 +1587,11 @@ and voice transmission.
 - Volume changes ramp smoothly over ~1s (no abrupt jumps)
 - Default volume: 50%
 - Titles resolved via `yt-dlp --flat-playlist` before playback
-- Audio pipeline: `yt-dlp | ffmpeg` subprocess, PCM fed to pymumble
+- Audio is downloaded before playback (`data/music/`); files are deleted
+  after playback unless `!keep` is used. Falls back to streaming on
+  download failure.
+- Audio pipeline: `ffmpeg` subprocess for local files, `yt-dlp | ffmpeg`
+  for streaming fallback, PCM fed to pymumble
 - Commands are Mumble-only; `!play` on other adapters replies with an error,
   other music commands silently no-op
 - Playback runs as an asyncio background task; the bot remains responsive
@@ -1657,3 +1663,17 @@ duck_floor = 1            # Floor volume % during ducking (default: 1)
 duck_silence = 15         # Seconds of silence before restoring (default: 15)
 duck_restore = 30         # Seconds for smooth volume restore (default: 30)
 ```
+
+### Download-First Playback
+
+Audio is downloaded to `data/music/` before playback begins. This
+eliminates CDN hiccups mid-stream and enables instant seeking. Files
+are identified by a hash of the URL so the same URL reuses the same
+file (natural dedup).
+
+- If download fails, playback falls back to streaming (`yt-dlp | ffmpeg`)
+- After a track finishes, the local file is automatically deleted
+- Use `!keep` during playback to preserve the file
+- Use `!kept` to list preserved files and their sizes
+- Use `!kept clear` to delete all preserved files
+- On cancel/error, files are not deleted (needed for `!resume`)