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/models.py

@@ -96,6 +96,10 @@ class AssetReference(Base):
     user_metadata: Mapped[dict[str, Any] | None] = mapped_column(
         JSON(none_as_null=True)
     )
+    system_metadata: Mapped[dict[str, Any] | None] = mapped_column(
+        JSON(none_as_null=True), nullable=True, default=None
+    )
+    prompt_id: Mapped[str | None] = mapped_column(String(36), nullable=True, default=None)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=False), nullable=False, default=get_utc_now
     )