Why

To be open-source ready, users must fork any repo they can read into their own namespace, hack on it, and open merge requests back. Core collaboration primitive — blocker for external contributors.

What

Add "Fork" action on the repository page. Forking creates a new repo under the actor's namespace that:

• Copies all git refs (branches + tags) from the source bare repo.
• Records a fork relationship (parent repo id) so the UI can show "forked from owner/name".
• Defaults visibility to source visibility (public source → public fork); never expose a private source through a fork.
• Is owned/writable by the forking user.

Scope

• Model: parent_repo_id (nullable FK) on the repository table + Flyway migration.
• Storage: server-side clone of the bare repo (local object copy, not full re-fetch over the network).
• UI: Fork button on repo header; fork picker if user already has a fork; "forked from …" link on forked repos.
• MR integration: merge requests can target the parent repo from a fork branch (cross-repo MR). May be split into a follow-up.
• MCP/API: forkRepository(owner, name) endpoint mirroring createRepository.
• Docs: docs/users/ (fork workflow), docs/admins/ (any new storage/config), docs/maintainers/ (fork relationship + storage design).

Out of scope

• Fork syncing / "update from upstream" — separate issue.
• Fork network graph visualization.

Acceptance

• [ ] User forks a public repo → new repo in their namespace with all refs.
• [ ] Fork page shows "forked from owner/name".
• [ ] Private source cannot be forked to a public repo by a non-authorized user.
• [ ] Failing test first, green after implementation.
• [ ] Docs updated in the same PR.