refactor: add explicit types to asset service functions

- Add typed result dataclasses: IngestResult, AddTagsResult,
  RemoveTagsResult, SetTagsResult, TagUsage
- Add UserMetadata type alias for user_metadata parameters
- Type helper functions with Session parameters
- Use TypedDicts at query layer to avoid circular imports
- Update manager.py and tests to use attribute access

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

app/assets/database/queries/__init__.py

@@ -37,6 +37,9 @@ from app.assets.database.queries.cache_state import (
     upsert_cache_state,
 )
 from app.assets.database.queries.tags import (
+    AddTagsDict,
+    RemoveTagsDict,
+    SetTagsDict,
     add_missing_tag_for_asset_id,
     add_tags_to_asset_info,
     bulk_insert_tags_and_meta,
@@ -90,4 +93,7 @@ __all__ = [
     "remove_missing_tag_for_asset_id",
     "list_tags_with_usage",
     "bulk_insert_tags_and_meta",
+    "AddTagsDict",
+    "RemoveTagsDict",
+    "SetTagsDict",
 ]