feat: live Gitea changelog, Gitea sidebar link, release title/body extraction

- Admin changelog page now fetches releases live from the public Gitea
  API (https://gitea.kalekber.cc) instead of a baked releases.json file
- Remove /static/releases.json from ui/.gitignore (no longer generated)
- Add Gitea to admin sidebar external tools (admin_nav_gitea i18n key,
  all 5 locales, compiled paraglide JS force-added)
- release.yaml: remove 'Fetch releases from Gitea API' step; extract
  release title and body from the tagged commit message for Gitea releases

ui/messages/en.json

@@ -368,6 +368,7 @@
   "admin_nav_logs": "Logs",
   "admin_nav_uptime": "Uptime",
   "admin_nav_push": "Push",
+  "admin_nav_gitea": "Gitea",
 
   "admin_scrape_status_idle": "Idle",
   "admin_scrape_status_running": "Running",