feat: non-destructive asset pruning with is_missing flag

- Add is_missing column to AssetCacheState for soft-delete
- Replace hard-delete pruning with mark_cache_states_missing_outside_prefixes
- Auto-restore missing cache states when files are re-scanned
- Filter out missing cache states from queries by default
- Rename functions for clarity:
  - mark_cache_states_missing_outside_prefixes (was delete_cache_states_outside_prefixes)
  - get_unreferenced_unhashed_asset_ids (was get_orphaned_seed_asset_ids)
  - mark_assets_missing_outside_prefixes (was prune_orphaned_assets)
  - mark_missing_outside_prefixes_safely (was prune_orphans_safely)
- Add restore_cache_states_by_paths for explicit restoration
- Add cleanup_unreferenced_assets for explicit hard-delete when needed
- Update API endpoint /api/assets/prune to use new soft-delete behavior

This preserves user metadata (tags, etc.) when base directories change,
allowing assets to be restored when the original paths become available again.

Amp-Thread-ID: https://ampcode.com/threads/T-019c3114-bf28-73a9-a4d2-85b208fd5462
Co-authored-by: Amp <amp@ampcode.com>

app/assets/services/__init__.py

@@ -11,7 +11,8 @@ from app.assets.services.asset_management import (
 from app.assets.services.bulk_ingest import (
     BulkInsertResult,
     batch_insert_seed_assets,
-    prune_orphaned_assets,
+    cleanup_unreferenced_assets,
+    mark_assets_missing_outside_prefixes,
 )
 from app.assets.services.file_utils import (
     get_mtime_ns,
@@ -77,7 +78,8 @@ __all__ = [
     "list_assets_page",
     "list_files_recursively",
     "list_tags",
-    "prune_orphaned_assets",
+    "cleanup_unreferenced_assets",
+    "mark_assets_missing_outside_prefixes",
     "remove_tags",
     "resolve_asset_for_download",
     "set_asset_preview",