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/openings_downloader/config.py

@@ -4,10 +4,6 @@ from pydantic_settings import BaseSettings
 class DownloaderSettings(BaseSettings):
     """Settings for the Openings Downloader module."""
 
-    # Shikimori API
-    shikimori_user_agent: str = "AnimeQuiz/1.0"
-    shikimori_token: str = ""  # Optional OAuth token for higher rate limits
-
     # S3 Storage limit (100 GB default)
     s3_storage_limit_bytes: int = 107_374_182_400  # 100 GB