Remove Shikimori API, use AnimeThemes only, switch to WebM format

- Remove ShikimoriService, use AnimeThemes API for search
- Replace shikimori_id with animethemes_slug as primary identifier
- Remove FFmpeg MP3 conversion, download WebM directly
- Add .webm support in storage and upload endpoints
- Update frontend to use animethemes_slug

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/main.py

@@ -126,6 +126,7 @@ async def get_audio(filename: str):
         ".wav": "audio/wav",
         ".ogg": "audio/ogg",
         ".m4a": "audio/mp4",
+        ".webm": "video/webm",
     }
     media_type = media_types.get(suffix, "audio/mpeg")
 
@@ -284,7 +285,7 @@ async def upload_audio(files: List[UploadFile] = File(...)):
     """Upload audio files to S3."""
     results = []
     for file in files:
-        if not file.filename.lower().endswith((".mp3", ".wav", ".ogg", ".m4a")):
+        if not file.filename.lower().endswith((".mp3", ".wav", ".ogg", ".m4a", ".webm")):
             results.append({"filename": file.filename, "success": False, "error": "Invalid file type"})
             continue