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>

backend/app/api/sources.py

@@ -4,11 +4,11 @@ from quart import Blueprint, jsonify, request
 
 from ..extensions import get_session
 from ..services.source_service import (
+    KNOWN_PLATFORMS,
     ArtistNotFoundError,
     DuplicateSourceError,
     EmptyUrlError,
     InvalidConfigError,
-    KNOWN_PLATFORMS,
     SourceService,
     UnknownPlatformError,
 )