fix: make !volume apply immediately during playback

stream_audio now accepts a callable for volume, re-read on each PCM
frame instead of capturing a static float at track start.

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

plugins/music.py

@@ -78,10 +78,11 @@ async def _play_loop(bot) -> None:
             done = asyncio.Event()
             ps["done_event"] = done
 
-            volume = ps["volume"] / 100.0
             try:
                 await bot.stream_audio(
-                    track.url, volume=volume, on_done=done,
+                    track.url,
+                    volume=lambda: ps["volume"] / 100.0,
+                    on_done=done,
                 )
             except asyncio.CancelledError:
                 raise
@@ -269,7 +270,7 @@ async def cmd_volume(bot, message):
 
     Usage:
         !volume         Show current volume
-        !volume <0-100> Set volume (applies on next track)
+        !volume <0-100> Set volume (takes effect immediately)
     """
     if not _is_mumble(bot):
         return