feat: add admin seeder and is_admin factory state

Runs is_admin migration, adds AdminSeeder for the admin user, registers
it in DatabaseSeeder, adds admin() factory state to UserFactory, and
adds AdminAccessTest covering both forbidden and ok cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Feature/Admin/AdminAccessTest.php

@@ -0,0 +1,20 @@
+<?php
+
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+
+uses(RefreshDatabase::class);
+
+it('denies non-admin users access to admin panel', function () {
+    $user = User::factory()->create();
+    $this->actingAs($user);
+
+    $this->get('/admin')->assertForbidden();
+});
+
+it('allows admin users to access admin panel', function () {
+    $user = User::factory()->admin()->create();
+    $this->actingAs($user);
+
+    $this->get('/admin')->assertOk();
+});