feat: add !resume to continue playback from last interruption

Tracks playback position via frame counting in stream_audio().
On stop/skip, saves URL + elapsed time to bot.state (SQLite).
!resume reloads the track and seeks to the saved position via
ffmpeg -ss. State persists across bot restarts.

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

docs/USAGE.md

@@ -1569,6 +1569,7 @@ and voice transmission.
 !play <query>        Search YouTube, play a random result
 !stop                Stop playback, clear queue
 !skip                Skip current track
+!resume              Resume last stopped/skipped track from saved position
 !queue               Show queue
 !queue <url>         Add to queue (alias for !play)
 !np                  Now playing
@@ -1589,3 +1590,5 @@ and voice transmission.
   other music commands silently no-op
 - Playback runs as an asyncio background task; the bot remains responsive
   to text commands during streaming
+- `!resume` continues from where playback was interrupted (`!stop`/`!skip`);
+  position is persisted via `bot.state` and survives bot restarts