split seeders by domain entity

Replaces the laravel-default scaffold seeder with one file per
domain (AdminUser, User, Post, Comment) plus a config/seed.php
to source admin credentials from env. DatabaseSeeder is now a
pure orchestrator that calls the sub-seeders in dependency
order.

backend/database/seeders/DatabaseSeeder.php

@@ -2,7 +2,6 @@
 
 namespace Database\Seeders;
 
-use App\Models\User;
 use Illuminate\Database\Console\Seeds\WithoutModelEvents;
 use Illuminate\Database\Seeder;
 
@@ -10,16 +9,13 @@ class DatabaseSeeder extends Seeder
 {
     use WithoutModelEvents;
 
-    /**
-     * Seed the application's database.
-     */
     public function run(): void
     {
-        // User::factory(10)->create();
-
-        User::factory()->create([
-            'name' => 'Test User',
-            'email' => 'test@example.com',
+        $this->call([
+            AdminUserSeeder::class,
+            UserSeeder::class,
+            PostSeeder::class,
+            CommentSeeder::class,
         ]);
     }
 }