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/AdminUserSeeder.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Database\Seeders;
+
+use App\Auth\PasswordHasher;
+use App\Shared\ValueObject\EmailAddress;
+use App\User\CreateUserDto;
+use App\User\User;
+use App\User\UserRepository;
+use DateTimeImmutable;
+use DateTimeZone;
+use Illuminate\Database\Seeder;
+
+class AdminUserSeeder extends Seeder
+{
+    public function __construct(
+        private UserRepository $userRepo,
+        private PasswordHasher $hasher,
+    ) {}
+
+    public function run(): void
+    {
+        $email = config('seed.admin.email', 'admin@tide.test');
+        $displayName = config('seed.admin.display_name', 'admin');
+        $password = config('seed.admin.password', 'changemenow');
+
+        $emailVo = new EmailAddress($email);
+        if ($this->userRepo->findByEmail($emailVo) !== null) {
+            return;
+        }
+
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
+        $created = $this->userRepo->create(new CreateUserDto(
+            email: $emailVo,
+            displayName: $displayName,
+            passwordHash: $this->hasher->hash($password),
+            isAdmin: false,
+            emailConfirmedAt: $now,
+        ));
+        $this->userRepo->update(new User(
+            id: $created->getId(),
+            email: $created->getEmail(),
+            displayName: $created->getDisplayName(),
+            passwordHash: $created->getPasswordHash(),
+            isAdmin: true,
+            emailConfirmedAt: $now,
+        ));
+    }
+}