refactor(assets): extract scanner logic into service modules

- Create file_utils.py with shared file utilities:
  - get_mtime_ns() - extract mtime in nanoseconds from stat
  - get_size_and_mtime_ns() - get both size and mtime
  - verify_file_unchanged() - check file matches DB mtime/size
  - list_files_recursively() - recursive directory listing

- Create bulk_ingest.py for bulk operations:
  - BulkInsertResult dataclass
  - batch_insert_seed_assets() - batch insert with conflict handling
  - prune_orphaned_assets() - clean up orphaned assets

- Update scanner.py to use new service modules instead of
  calling database queries directly

- Update ingest.py to use shared get_size_and_mtime_ns()

- Export new functions from services/__init__.py

Amp-Thread-ID: https://ampcode.com/threads/T-019c2ae7-f701-716a-a0dd-1feb988732fb
Co-authored-by: Amp <amp@ampcode.com>

app/assets/services/__init__.py

@@ -8,6 +8,17 @@ from app.assets.services.asset_management import (
     set_asset_preview,
     update_asset_metadata,
 )
+from app.assets.services.bulk_ingest import (
+    BulkInsertResult,
+    batch_insert_seed_assets,
+    prune_orphaned_assets,
+)
+from app.assets.services.file_utils import (
+    get_mtime_ns,
+    get_size_and_mtime_ns,
+    list_files_recursively,
+    verify_file_unchanged,
+)
 from app.assets.services.ingest import (
     DependencyMissingError,
     HashMismatchError,
@@ -39,29 +50,15 @@ from app.assets.services.tagging import (
 )
 
 __all__ = [
-    "ingest_file_from_path",
-    "register_existing_asset",
-    "upload_from_temp_path",
-    "create_from_hash",
-    "HashMismatchError",
-    "DependencyMissingError",
-    "asset_exists",
-    "get_asset_by_hash",
-    "get_asset_detail",
-    "list_assets_page",
-    "resolve_asset_for_download",
-    "update_asset_metadata",
-    "delete_asset_reference",
-    "set_asset_preview",
-    "apply_tags",
-    "remove_tags",
-    "list_tags",
     "AddTagsResult",
     "AssetData",
     "AssetDetailResult",
     "AssetInfoData",
     "AssetSummaryData",
+    "BulkInsertResult",
+    "DependencyMissingError",
     "DownloadResolutionResult",
+    "HashMismatchError",
     "IngestResult",
     "ListAssetsResult",
     "RegisterAssetResult",
@@ -70,4 +67,25 @@ __all__ = [
     "TagUsage",
     "UploadResult",
     "UserMetadata",
+    "apply_tags",
+    "asset_exists",
+    "batch_insert_seed_assets",
+    "create_from_hash",
+    "delete_asset_reference",
+    "get_asset_by_hash",
+    "get_asset_detail",
+    "get_mtime_ns",
+    "get_size_and_mtime_ns",
+    "ingest_file_from_path",
+    "list_assets_page",
+    "list_files_recursively",
+    "list_tags",
+    "prune_orphaned_assets",
+    "register_existing_asset",
+    "remove_tags",
+    "resolve_asset_for_download",
+    "set_asset_preview",
+    "update_asset_metadata",
+    "upload_from_temp_path",
+    "verify_file_unchanged",
 ]