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/services/__init__.py

@@ -1,11 +1,9 @@
 # Services for Openings Downloader
-from .shikimori import ShikimoriService
 from .animethemes import AnimeThemesService
 from .downloader import DownloadService
 from .storage_tracker import StorageTrackerService
 
 __all__ = [
-    "ShikimoriService",
     "AnimeThemesService",
     "DownloadService",
     "StorageTrackerService",