feat(admin): build attach-to-entity picker UI on FormDetailPage

[cdcore09]

Summary

FormDetailPage currently displays the attached entity (event/announcement/group) and offers a Detach action, but there's no UI to attach a form to an entity if it's currently unattached. v1 ships with admins setting entityType/entityId via the API.

Requirements

• Build a typeahead/picker that lets admin browse events, announcements, or groups and select one to attach
• On selection, PATCH /admin/forms/:id with { entityType, entityId }
• Respect the DB CHECK constraint (both fields must end up set or both null)
• Validation: if form's scope is group, surface a warning when attaching to a non-group entity

Context

Plan 4 noted this as deferred. Now that forms ship, the API path works but admins need the UI to use it.

Files

• apps/admin/src/pages/forms/FormDetailPage.tsx (Identity tab)
• New helper: apps/admin/src/hooks/useArtifactPicker.ts (consumes /admin/events, /admin/announcements, /admin/groups for typeahead)