feat: embed album art in ID3 tags on download

APIC tag now embeds the track's artwork (if it exists) into the MP3
file when downloading. Supports PNG, JPEG, and WebP. Key and BPM were
already being written — this was the missing piece.

backend/src/main.py

@@ -135,7 +135,7 @@ async def stream_audio(clip_id: str):
     return FileResponse(file_path, media_type="audio/mpeg", filename=clip["filename"])
 
 from mutagen.mp3 import MP3
-from mutagen.id3 import ID3, TIT1, TIT2, TPE1, TALB, COMM, TBPM, TKEY
+from mutagen.id3 import ID3, TIT1, TIT2, TPE1, TALB, COMM, TBPM, TKEY, APIC, PictureType
 
 @app.get("/api/clips/{clip_id}/download")
 async def download_clip(clip_id: str):
@@ -197,6 +197,21 @@ async def download_clip(clip_id: str):
             if clip.get("key"):
                 audio.tags["TKEY"] = TKEY(encoding=3, text=clip["key"])
             
+            # Album art
+            if clip.get("artwork_filename"):
+                artwork_path = ARTWORK_DIR / clip["artwork_filename"]
+                if artwork_path.exists():
+                    artwork_data = artwork_path.read_bytes()
+                    ext = artwork_path.suffix.lower()
+                    mime = {'.png': 'image/png', '.jpg': 'image/jpeg', '.jpeg': 'image/jpeg', '.webp': 'image/webp'}.get(ext, 'image/jpeg')
+                    audio.tags["APIC"] = APIC(
+                        encoding=3,
+                        mime=mime,
+                        type=PictureType.COVER_FRONT,
+                        desc='Cover',
+                        data=artwork_data
+                    )
+            
             # Also store the prompt in a custom tag for reference
             if clip.get("prompt"):
                 audio.tags["TIT1"] = TIT1(encoding=3, text=clip["prompt"][:100])