Open WebUI: Jupyter code execution works despite `ENABLE_CODE_EXECUTION=false` — feature gate bypassed

Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.8.12, the /api/v1/utils/code/execute endpoint executes arbitrary Python code via Jupyter for any verified user, even when the admin has set ENABLE_CODE_EXECUTION=false. The feature gate is not enforced on the API endpoint — the configuration says "disabled" but code still executes. This vulnerability is fixed in 0.8.12.