fix: download track on !keep when local file is missing

When the initial download failed during playback and the track streamed
directly from URL, !keep would refuse with "No local file". Now it
downloads the track on the spot before keeping it.

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

plugins/music.py

@@ -1405,8 +1405,20 @@ async def cmd_keep(bot, message):
         await bot.reply(message, "Nothing playing")
         return
     if track.local_path is None:
-        await bot.reply(message, "No local file for current track")
-        return
+        if not track.url:
+            await bot.reply(message, "No local file for current track")
+            return
+        # Download on the spot -- track was streaming without a local file
+        loop = asyncio.get_running_loop()
+        tid = hashlib.md5(track.url.encode()).hexdigest()[:12]
+        dl_path = await loop.run_in_executor(
+            None, _download_track, track.url, tid, track.title,
+        )
+        if dl_path:
+            track.local_path = dl_path
+        else:
+            await bot.reply(message, "Download failed, cannot keep track")
+            return
     track.keep = True
 
     # Check if this track is already kept (by normalized URL)