feat: platform admin member management — invite, remove, role update

Add member management to the platform admin organisation detail page:
- Backend: invite (creates invitation or directly adds existing user),
  remove member, update member role endpoints on AdminOrganisationController
- Backend: show endpoint now returns members alongside organisation data
- Frontend: members table with inline role editing, invite dialog,
  remove confirmation dialog on /platform/organisations/[id]
- Tests: 7 new tests covering happy paths and edge cases (self-removal,
  existing member, non-super_admin denied)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/routes/api.php

@@ -90,6 +90,9 @@ Route::prefix('admin')
     ->group(function () {
         // Organisations
         Route::apiResource('organisations', AdminOrganisationController::class);
+        Route::post('organisations/{organisation}/invite', [AdminOrganisationController::class, 'invite']);
+        Route::put('organisations/{organisation}/members/{user}', [AdminOrganisationController::class, 'updateMemberRole']);
+        Route::delete('organisations/{organisation}/members/{user}', [AdminOrganisationController::class, 'removeMember']);
 
         // Users
         Route::apiResource('users', AdminUserController::class)