docs: document seek command and volume persistence

docs/USAGE.md

@@ -1569,11 +1569,12 @@ and voice transmission.
 !play <query>        Search YouTube, play a random result
 !stop                Stop playback, clear queue
 !skip                Skip current track
+!seek <offset>       Seek to position (1:30, 90, +30, -30)
 !resume              Resume last stopped/skipped track from saved position
 !queue               Show queue
 !queue <url>         Add to queue (alias for !play)
 !np                  Now playing
-!volume [0-100]      Get/set volume
+!volume [0-100]      Get/set volume (persisted across restarts)
 !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 +1586,7 @@ and voice transmission.
 - Playlists are expanded into individual tracks; excess tracks are
   truncated at the queue limit
 - Volume changes ramp smoothly over ~1s (no abrupt jumps)
-- Default volume: 50%
+- Default volume: 50%; persisted via `bot.state` across restarts
 - Titles resolved via `yt-dlp --flat-playlist` before playback
 - Audio is downloaded before playback (`data/music/`); files are deleted
   after playback unless `!keep` is used. Falls back to streaming on
@@ -1614,6 +1615,23 @@ automatically resumes playback -- but only after the channel is silent
 - Chat messages announce resume intentions and abort reasons
 - The reconnect watcher starts via the `on_connected` plugin lifecycle hook
 
+### Seeking
+
+Fast-forward or rewind within the currently playing track.
+
+```
+!seek 1:30       Seek to 1 minute 30 seconds
+!seek 90         Seek to 90 seconds
+!seek +30        Jump forward 30 seconds
+!seek -30        Jump backward 30 seconds
+!seek +1:00      Jump forward 1 minute
+```
+
+- Absolute offsets (`1:30`, `90`) seek to that position from the start
+- Relative offsets (`+30`, `-1:00`) jump from the current position
+- Negative seeks are clamped to the start of the track
+- Seeking restarts the audio pipeline at the new position
+
 ### Disconnect-Resilient Streaming
 
 During brief network disconnects (~5-15s), the audio stream stays alive.