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>

main.py

@@ -7,7 +7,7 @@ import folder_paths
 import time
 from comfy.cli_args import args, enables_dynamic_vram
 from app.logger import setup_logger
-from app.assets.services.scanner import seed_assets
+from app.assets.scanner import seed_assets
 import itertools
 import utils.extra_config
 import logging