Replace removed upstream Gitea update checker with MokoGitea-native
version that checks our own releases API.
- New module: modules/updatechecker/ — fetches latest release from
git.mokoconsulting.tech, compares semver, caches result
- Cron task: runs every 24h (and at startup)
- Admin dashboard: shows green banner when update available
- Configurable via [update_checker] in app.ini
Closes#74
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>