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/db_models.py

@@ -27,12 +27,11 @@ class DownloadStatus(str, enum.Enum):
 
 
 class Anime(Base):
-    """Anime entity from Shikimori."""
+    """Anime entity from AnimeThemes."""
     __tablename__ = "anime"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
-    shikimori_id: Mapped[int] = mapped_column(Integer, unique=True, index=True, nullable=False)
-    animethemes_slug: Mapped[Optional[str]] = mapped_column(String(255), nullable=True, index=True)
+    animethemes_slug: Mapped[str] = mapped_column(String(255), unique=True, index=True, nullable=False)
 
     title_russian: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
     title_english: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
@@ -54,7 +53,7 @@ class Anime(Base):
     )
 
     def __repr__(self):
-        return f"<Anime {self.shikimori_id}: {self.title_russian or self.title_english}>"
+        return f"<Anime {self.animethemes_slug}: {self.title_english}>"
 
 
 class AnimeTheme(Base):