refactor: remove GitLab CI config and docs after full Gitea migration

- Delete .gitlab-ci.yml (replaced by .gitea/workflows/ci.yml)
- Delete docs/deployment/gitlab.md (superseded by gitea.md)
- Update audit rules to reference .gitea/workflows/*.yml
- Update validate_audit.py to check Gitea CI paths
- Clean up GitLab references in gitea.md, mkdocs.yml, .dockerignore
- Mark IPv6 AAAA records as completed in hetzner docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mkdocs.yml

@@ -212,7 +212,6 @@ nav:
       - Traditional VPS: deployment/production.md
       - Docker: deployment/docker.md
       - CloudFlare Setup: deployment/cloudflare.md
-      - GitLab CI/CD: deployment/gitlab.md
       - Gitea CI/CD: deployment/gitea.md
       - Hetzner Server Setup: deployment/hetzner-server-setup.md
       - Environment Variables: deployment/environment.md