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/database/queries/__init__.py

@@ -28,12 +28,13 @@ from app.assets.database.queries.cache_state import (
     bulk_set_needs_verify,
     delete_assets_by_ids,
     delete_cache_states_by_ids,
-    delete_cache_states_outside_prefixes,
     delete_orphaned_seed_asset,
     get_cache_states_by_paths_and_asset_ids,
     get_cache_states_for_prefixes,
-    get_orphaned_seed_asset_ids,
+    get_unreferenced_unhashed_asset_ids,
     list_cache_states_by_asset_id,
+    mark_cache_states_missing_outside_prefixes,
+    restore_cache_states_by_paths,
     upsert_cache_state,
 )
 from app.assets.database.queries.tags import (
@@ -68,7 +69,6 @@ __all__ = [
     "delete_asset_info_by_id",
     "delete_assets_by_ids",
     "delete_cache_states_by_ids",
-    "delete_cache_states_outside_prefixes",
     "delete_orphaned_seed_asset",
     "ensure_tags_exist",
     "fetch_asset_info_and_asset",
@@ -80,13 +80,15 @@ __all__ = [
     "get_cache_states_by_paths_and_asset_ids",
     "get_cache_states_for_prefixes",
     "get_or_create_asset_info",
-    "get_orphaned_seed_asset_ids",
+    "get_unreferenced_unhashed_asset_ids",
     "insert_asset_info",
     "list_asset_infos_page",
     "list_cache_states_by_asset_id",
     "list_tags_with_usage",
+    "mark_cache_states_missing_outside_prefixes",
     "remove_missing_tag_for_asset_id",
     "remove_tags_from_asset_info",
+    "restore_cache_states_by_paths",
     "set_asset_info_metadata",
     "set_asset_info_preview",
     "set_asset_info_tags",