feat(assets): align local API with cloud spec

Unify response models, add missing fields, and align input schemas with
the cloud OpenAPI spec at cloud.comfy.org/openapi.

- Replace AssetSummary/AssetDetail/AssetUpdated with single Asset model
- Add is_immutable, metadata (system_metadata), prompt_id fields
- Support mime_type and preview_id in update endpoint
- Make CreateFromHashBody.name optional, add mime_type, require >=1 tag
- Add id/mime_type/preview_id to upload, relax tags to optional
- Rename total_tags → tags in tag add/remove responses
- Add GET /api/assets/tags/refine histogram endpoint
- Add DB migration for system_metadata and prompt_id columns

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/assets/database/queries/__init__.py

@@ -54,6 +54,7 @@ from app.assets.database.queries.tags import (
     bulk_insert_tags_and_meta,
     ensure_tags_exist,
     get_reference_tags,
+    list_tag_counts_for_filtered_assets,
     list_tags_with_usage,
     remove_missing_tag_for_asset_id,
     remove_tags_from_reference,
@@ -99,6 +100,7 @@ __all__ = [
     "insert_reference",
     "list_references_by_asset_id",
     "list_references_page",
+    "list_tag_counts_for_filtered_assets",
     "list_tags_with_usage",
     "mark_references_missing_outside_prefixes",
     "reassign_asset_references",