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/UserSeeder.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Database\Seeders;
+
+use App\Auth\PasswordHasher;
+use App\Shared\ValueObject\EmailAddress;
+use App\User\CreateUserDto;
+use App\User\UserRepository;
+use DateTimeImmutable;
+use DateTimeZone;
+use Illuminate\Database\Seeder;
+
+class UserSeeder extends Seeder
+{
+    public function __construct(
+        private UserRepository $userRepo,
+        private PasswordHasher $hasher,
+    ) {}
+
+    public function run(): void
+    {
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
+        $samples = [
+            ['email' => 'alice@example.com', 'displayName' => 'alice'],
+            ['email' => 'bob@example.com', 'displayName' => 'bob'],
+            ['email' => 'carol@example.com', 'displayName' => 'carol'],
+        ];
+        foreach ($samples as $sample) {
+            $email = new EmailAddress($sample['email']);
+            if ($this->userRepo->findByEmail($email) !== null) {
+                continue;
+            }
+            $this->userRepo->create(new CreateUserDto(
+                email: $email,
+                displayName: $sample['displayName'],
+                passwordHash: $this->hasher->hash('longenoughpassword'),
+                isAdmin: false,
+                emailConfirmedAt: $now,
+            ));
+        }
+    }
+}