Quick wins: Lower default volume to 25% and add file upload support

- Scale volume by 0.25x to prevent earrape (user still sees 0-200%)
  - Add support for direct audio file URL links (.mp3, .mp4, etc.)
  - New =playfile command for Discord file uploads
  - Supports MP3, MP4, WAV, OGG, FLAC, M4A, WEBM, AAC, OPUS formats

cogs/music/queue.py

@@ -497,7 +497,10 @@ async def play(ctx):
         return
 
     try:
-        vol = await get_volume(server_id) / 100.0
+        # Scale volume down to prevent earrape
+        # User sees 0-200%, but internally we scale by 0.25
+        # So user's 100% = 0.25 actual volume (25%)
+        vol = await get_volume(server_id) / 100.0 * 0.25
         fx = await get_effect(server_id)
         opts = get_effect_options(fx)