feat(api): Update anime API endpoints to use database storage

Task 6: Update Anime API endpoints to use database
- Modified add_series endpoint to save series to database when available
- Added get_optional_database_session dependency for graceful fallback
- Falls back to file-based storage when database unavailable
- All 55 API tests and 809 unit tests pass

src/server/utils/dependencies.py

@@ -134,6 +134,38 @@ async def get_database_session() -> AsyncGenerator:
         )
 
 
+async def get_optional_database_session() -> AsyncGenerator:
+    """
+    Dependency to get optional database session.
+    
+    Unlike get_database_session(), this returns None if the database
+    is not available, allowing endpoints to fall back to other storage.
+    
+    Yields:
+        AsyncSession or None: Database session if available, None otherwise
+    
+    Example:
+        @app.post("/anime/add")
+        async def add_anime(
+            db: Optional[AsyncSession] = Depends(get_optional_database_session)
+        ):
+            if db:
+                # Use database
+                await AnimeSeriesService.create(db, ...)
+            else:
+                # Fall back to file-based storage
+                series_app.list.add(serie)
+    """
+    try:
+        from src.server.database import get_db_session
+        
+        async with get_db_session() as session:
+            yield session
+    except (ImportError, RuntimeError):
+        # Database not available - yield None
+        yield None
+
+
 def get_current_user(
     credentials: Optional[HTTPAuthorizationCredentials] = Depends(
         http_bearer_security