refactor: move scanner.py out of services to top-level assets module

Scanner is used externally by main.py and server.py for startup/maintenance,
not as part of the regular service layer. Moving it to app/assets/scanner.py
makes the public API clearer.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/assets/api/routes.py

@@ -17,7 +17,7 @@ from app.assets.api.schemas_in import (
     UploadError,
 )
 from app.assets.api.upload import parse_multipart_upload
-from app.assets.services.scanner import seed_assets as scanner_seed_assets
+from app.assets.scanner import seed_assets as scanner_seed_assets
 from typing import Any