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/app/api/endpoints/session_branches.py

@@ -127,6 +127,7 @@ async def create_fork(
     )
 
     await db.commit()
+    await db.refresh(fork_point)
     return ForkPointResponse.model_validate(fork_point)