Fix generator exception handling in database dependencies

- Add proper exception handling in get_database_session and get_optional_database_session
- Prevents 'generator didn't stop after athrow()' error when HTTPException is raised
- Add mock for BackgroundLoaderService in anime endpoint tests
- Update test expectations to match 202 Accepted response for async add_series endpoint

src/server/utils/dependencies.py

@@ -126,7 +126,12 @@ async def get_database_session() -> AsyncGenerator:
         from src.server.database import get_db_session
         
         async with get_db_session() as session:
-            yield session
+            try:
+                yield session
+            except Exception:
+                # Re-raise the exception to let FastAPI handle it
+                # This prevents "generator didn't stop after athrow()" error
+                raise
     except ImportError:
         raise HTTPException(
             status_code=status.HTTP_501_NOT_IMPLEMENTED,
@@ -165,7 +170,12 @@ async def get_optional_database_session() -> AsyncGenerator:
         from src.server.database import get_db_session
         
         async with get_db_session() as session:
-            yield session
+            try:
+                yield session
+            except Exception:
+                # Re-raise the exception to let FastAPI handle it
+                # This prevents "generator didn't stop after athrow()" error
+                raise
     except (ImportError, RuntimeError):
         # Database not available - yield None
         yield None