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>

tests-unit/assets_test/test_tags_api.py

@@ -97,7 +97,7 @@ def test_add_and_remove_tags(http: requests.Session, api_base: str, seeded_asset
     # normalized, deduplicated; 'unit-tests' was already present from the seed
     assert set(b1["added"]) == {"newtag", "beta"}
     assert set(b1["already_present"]) == {"unit-tests"}
-    assert "newtag" in b1["total_tags"] and "beta" in b1["total_tags"]
+    assert "newtag" in b1["tags"] and "beta" in b1["tags"]
 
     rg = http.get(f"{api_base}/api/assets/{aid}", timeout=120)
     g = rg.json()