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>

alembic_db/versions/0003_add_metadata_prompt.py

@@ -0,0 +1,31 @@
+"""
+Add system_metadata and prompt_id columns to asset_references.
+
+Revision ID: 0003_add_metadata_prompt
+Revises: 0002_merge_to_asset_references
+Create Date: 2026-03-09
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0003_add_metadata_prompt"
+down_revision = "0002_merge_to_asset_references"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("asset_references") as batch_op:
+        batch_op.add_column(
+            sa.Column("system_metadata", sa.JSON(), nullable=True)
+        )
+        batch_op.add_column(
+            sa.Column("prompt_id", sa.String(length=36), nullable=True)
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("asset_references") as batch_op:
+        batch_op.drop_column("prompt_id")
+        batch_op.drop_column("system_metadata")