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>

database/seeders/AdminSeeder.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Database\Seeders;
+
+use App\Models\User;
+use Illuminate\Database\Seeder;
+use Illuminate\Support\Facades\Hash;
+
+class AdminSeeder extends Seeder
+{
+    public function run(): void
+    {
+        User::updateOrCreate(
+            ['email' => 'uovidiu@sent.com'],
+            [
+                'name' => 'Ovidiu U',
+                'password' => Hash::make('changeme'),
+                'is_admin' => true,
+            ]
+        );
+    }
+}