from pathlib import Path
p = Path(r'C:\kefu\wechat_rpa\admin_backend.py')
s = p.read_text(encoding='utf-8')
needle = '        Database._ensure_model_calls_task_unique(db)'
assert needle in s and 'idx_model_calls_usage_utc' not in s
s = s.replace(needle, '''        db.execute("CREATE INDEX IF NOT EXISTS idx_model_calls_usage_utc "
                   "ON model_calls(datetime(created_at), tenant_id, purpose)")
''' + needle, 1)
p.write_text(s, encoding='utf-8', newline='')