fix: add server_default to script_template columns, refresh fork_point after commit

- script_template.py: add server_default for requires_elevation,
  is_gallery_featured, gallery_sort_order so Base.metadata.create_all
  emits proper SQL DEFAULTs (test fixtures use raw SQL INSERT)
- session_branches.py: refresh fork_point after commit so JSONB options
  field is loaded before Pydantic serialization
- test_session_branches_api.py: add status assertion on fork response

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/test_session_branches_api.py

@@ -106,6 +106,7 @@ async def test_switch_branch(client: AsyncClient, test_user, auth_headers, test_
             ],
         },
     )
+    assert fork_resp.status_code == 201, fork_resp.text
     fork_data = fork_resp.json()
     branch_b_id = fork_data["options"][1]["branch_id"]