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/CommentSeeder.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace Database\Seeders;
+
+use App\Comment\CommentRepository;
+use App\Comment\CreateCommentDto;
+use App\Post\PostRepository;
+use App\Shared\ValueObject\EmailAddress;
+use App\User\UserRepository;
+use DateTimeImmutable;
+use DateTimeZone;
+use Illuminate\Database\Seeder;
+
+class CommentSeeder extends Seeder
+{
+    public function __construct(
+        private CommentRepository $commentRepo,
+        private PostRepository $postRepo,
+        private UserRepository $userRepo,
+    ) {}
+
+    public function run(): void
+    {
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
+        $bob = $this->userRepo->findByEmail(
+            new EmailAddress('bob@example.com'),
+        );
+        $carol = $this->userRepo->findByEmail(
+            new EmailAddress('carol@example.com'),
+        );
+        if ($bob === null || $carol === null) {
+            return;
+        }
+        $posts = $this->postRepo->findRecent(10);
+        if (count($posts) === 0) {
+            return;
+        }
+        $firstPost = $posts[0];
+
+        $this->commentRepo->create(new CreateCommentDto(
+            postId: $firstPost->getId(),
+            userId: $bob->getId(),
+            body: 'Welcome aboard!',
+            createdAt: $now,
+        ));
+        $this->commentRepo->create(new CreateCommentDto(
+            postId: $firstPost->getId(),
+            userId: $carol->getId(),
+            body: 'Looking forward to more.',
+            createdAt: $now->modify('+1 minute'),
+        ));
+    }
+}