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/schemas.py

@@ -5,14 +5,12 @@ from pydantic import BaseModel, Field
 from .db_models import ThemeType, DownloadStatus
 
 
-# ============== Shikimori Search ==============
+# ============== AnimeThemes Search ==============
 
 class AnimeSearchResult(BaseModel):
-    """Single anime search result from Shikimori."""
-    shikimori_id: int
-    title_russian: Optional[str] = None
+    """Single anime search result from AnimeThemes."""
+    animethemes_slug: str
     title_english: Optional[str] = None
-    title_japanese: Optional[str] = None
     year: Optional[int] = None
     poster_url: Optional[str] = None
 
@@ -48,10 +46,8 @@ class ThemeInfo(BaseModel):
 class AnimeDetailResponse(BaseModel):
     """Detailed anime info with themes."""
     id: int
-    shikimori_id: int
-    title_russian: Optional[str] = None
+    animethemes_slug: str
     title_english: Optional[str] = None
-    title_japanese: Optional[str] = None
     year: Optional[int] = None
     poster_url: Optional[str] = None
     themes: List[ThemeInfo]