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/api/upload.py

@@ -52,6 +52,9 @@ async def parse_multipart_upload(
     user_metadata_raw: str | None = None
     provided_hash: str | None = None
     provided_hash_exists: bool | None = None
+    provided_id: str | None = None
+    provided_mime_type: str | None = None
+    provided_preview_id: str | None = None
 
     file_written = 0
     tmp_path: str | None = None
@@ -128,6 +131,12 @@ async def parse_multipart_upload(
             provided_name = (await field.text()) or None
         elif fname == "user_metadata":
             user_metadata_raw = (await field.text()) or None
+        elif fname == "id":
+            provided_id = ((await field.text()) or "").strip() or None
+        elif fname == "mime_type":
+            provided_mime_type = ((await field.text()) or "").strip() or None
+        elif fname == "preview_id":
+            provided_preview_id = ((await field.text()) or "").strip() or None
 
     if not file_present and not (provided_hash and provided_hash_exists):
         raise UploadError(
@@ -152,6 +161,9 @@ async def parse_multipart_upload(
         user_metadata_raw=user_metadata_raw,
         provided_hash=provided_hash,
         provided_hash_exists=provided_hash_exists,
+        provided_id=provided_id,
+        provided_mime_type=provided_mime_type,
+        provided_preview_id=provided_preview_id,
     )