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/storage.py

@@ -117,7 +117,7 @@ class S3Storage:
 
     def list_audio_files(self) -> list[str]:
         """List available audio files."""
-        return self.list_files("audio/", [".mp3", ".wav", ".ogg", ".m4a"])
+        return self.list_files("audio/", [".mp3", ".wav", ".ogg", ".m4a", ".webm"])
 
     def list_background_videos(self) -> list[str]:
         """List available background videos."""
@@ -167,7 +167,15 @@ class S3Storage:
 
     def upload_audio(self, filename: str, file_data: bytes) -> bool:
         """Upload audio file to S3."""
-        content_type = "audio/mpeg" if filename.lower().endswith(".mp3") else "audio/wav"
+        ext = filename.lower().split(".")[-1]
+        content_types = {
+            "mp3": "audio/mpeg",
+            "wav": "audio/wav",
+            "ogg": "audio/ogg",
+            "m4a": "audio/mp4",
+            "webm": "video/webm",
+        }
+        content_type = content_types.get(ext, "audio/mpeg")
         return self.upload_file(f"audio/{filename}", file_data, content_type)
 
     def upload_background(self, filename: str, file_data: bytes) -> bool: