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/app/Http/Requests/Admin/AdminInviteMemberRequest.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Admin;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+final class AdminInviteMemberRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        return [
+            'email' => ['required', 'email', 'max:255'],
+            'role' => ['required', 'in:org_admin,org_member'],
+        ];
+    }
+}

api/app/Http/Requests/Admin/AdminUpdateMemberRoleRequest.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Admin;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+final class AdminUpdateMemberRoleRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        return [
+            'role' => ['required', 'in:org_admin,org_member'],
+        ];
+    }
+}