fix(fc3a): ruff I001 import-block fixes + use sqlalchemy.case (not func.case)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_source_service.py

@@ -3,11 +3,11 @@ from sqlalchemy import select
 
 from backend.app.models import Artist
 from backend.app.services.source_service import (
+    KNOWN_PLATFORMS,
     ArtistNotFoundError,
     DuplicateSourceError,
     EmptyUrlError,
     InvalidConfigError,
-    KNOWN_PLATFORMS,
     SourceService,
     UnknownPlatformError,
 )