storage: add UploadSessions columns for server-proxied upload metadata

The server-proxied upload flow (replaces presign-PUT) stores the
file-level metadata directly on the UploadSessions row at session
creation, so step 2 (PUT bytes) can build the StorageObjects row
without re-sending metadata through a separate CompleteUploadRequest.

New columns on upload_sessions:
  file_name    VARCHAR(255) NOT NULL DEFAULT ''
  usage_type   VARCHAR(32)  NOT NULL DEFAULT 'working_copy'
  visibility   VARCHAR(16)  NOT NULL DEFAULT 'private'
  is_immutable TINYINT(1)   NOT NULL DEFAULT 0

The SQLAlchemy model already declares these columns; this migration
applies the schema change to MySQL.

Migration: c3d4e5f6a7b8_upload_session_object_metadata.py
(chains off a2b3c4d5e6f7)

migrations/versions/c3d4e5f6a7b8_upload_session_object_metadata.py

@@ -0,0 +1,73 @@
+"""add object metadata columns to upload_sessions
+
+Stores file_name / usage_type / visibility / is_immutable at session creation
+so the server-proxied PUT step can build the StorageObjects row without
+re-sending them. Replaces the old CompleteUploadRequest payload that bridged
+the presign-PUT and head()-validate steps.
+
+Revision ID: c3d4e5f6a7b8
+Revises: a2b3c4d5e6f7
+Create Date: 2026-08-05 12:00:00
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "c3d4e5f6a7b8"
+down_revision: str | Sequence[str] | None = "a2b3c4d5e6f7"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "upload_sessions",
+        sa.Column(
+            "file_name",
+            sa.String(length=255),
+            nullable=False,
+            server_default="",
+        ),
+    )
+    op.add_column(
+        "upload_sessions",
+        sa.Column(
+            "usage_type",
+            sa.String(length=32),
+            nullable=False,
+            server_default="working_copy",
+            comment=(
+                "data_resource/version_artifact/snapshot/run_log/run_result/"
+                "working_copy/public_script"
+            ),
+        ),
+    )
+    op.add_column(
+        "upload_sessions",
+        sa.Column(
+            "visibility",
+            sa.String(length=16),
+            nullable=False,
+            server_default="private",
+            comment="private/workspace/public",
+        ),
+    )
+    op.add_column(
+        "upload_sessions",
+        sa.Column(
+            "is_immutable",
+            sa.TINYINT(1),
+            nullable=False,
+            server_default="0",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("upload_sessions", "is_immutable")
+    op.drop_column("upload_sessions", "visibility")
+    op.drop_column("upload_sessions", "usage_type")
+    op.drop_column("upload_sessions", "file_name")