Change RENOVATE_ENDPOINT from /api/v3 to /api/v1. Gitea uses v1 for all
API routes; the /api/v3 path returns 404 which Renovate surfaces as an
authentication failure.
The runner environment does not have Node.js in PATH, causing
actions/checkout@v4 and renovatebot/github-action@v41.0.0 (both JS
actions) to fail at startup.
Use renovate/renovate:latest as the job container and invoke the
renovate binary directly via run:, consistent with how all other
workflows in this repo handle checkout and tooling. The Checkout step
was also unnecessary — Renovate manages its own git operations.
