Repo moved from GitLab to Gitea (git.unbound.se). Updates Go module path, all internal Go imports, Renovate registry pkg name, .buildtools.yaml registry block, and Dockerfile (drops obsolete GOPRIVATE + GITLAB_USER/GITLAB_TOKEN).
Reviewed-on: #307
Refactor the application to a Go-based architecture for improved
performance and maintainability. Replace the Dockerfile to utilize a
multi-stage build process, enhancing image efficiency. Implement
comprehensive session store tests to ensure reliability and create
new OAuth handlers for managing authentication efficiently. Update
documentation to reflect these structural changes.