0fed6a42 — set / change / clear a project's parent (subproject hierarchy).

REST mirror of the set_parent_project MCP tool, wired for the dashboard's "Make subproject of…" / "Detach from parent" kebab actions. Body: {"parent_project_id": "<id>"} to nest, or {"parent_project_id": null} to detach to top level.

Validation lives in db.set_parent_project and enforces the one-level-deep invariant (a project cannot be its own parent, the parent must itself be top-level, and a project that already has subprojects cannot become one). A violation raises ValueError → 400; an unknown project_id → 404. Broadcasts a project_parent_changed WS event so other open dashboards re-render their sidebar tree.