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/PostSeeder.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Database\Seeders;
+
+use App\Post\CreatePostDto;
+use App\Post\PostRepository;
+use App\Shared\ValueObject\EmailAddress;
+use App\User\UserRepository;
+use DateTimeImmutable;
+use DateTimeZone;
+use Illuminate\Database\Seeder;
+
+class PostSeeder extends Seeder
+{
+    public function __construct(
+        private PostRepository $postRepo,
+        private UserRepository $userRepo,
+    ) {}
+
+    public function run(): void
+    {
+        $now = new DateTimeImmutable('now', new DateTimeZone('UTC'));
+        $samples = [
+            [
+                'email' => 'alice@example.com',
+                'title' => 'Hello World',
+                'body' => 'My first post on TIDE.',
+            ],
+            [
+                'email' => 'bob@example.com',
+                'title' => 'Riding the wave',
+                'body' => 'Bob shares his thoughts.',
+            ],
+            [
+                'email' => 'carol@example.com',
+                'title' => 'Notes from week one',
+                'body' => 'Carol lays out her plan.',
+            ],
+        ];
+        foreach ($samples as $index => $sample) {
+            $author = $this->userRepo->findByEmail(
+                new EmailAddress($sample['email']),
+            );
+            if ($author === null) {
+                continue;
+            }
+            $this->postRepo->create(new CreatePostDto(
+                userId: $author->getId(),
+                title: $sample['title'],
+                body: $sample['body'],
+                createdAt: $now->modify("-{$index} hour"),
+            ));
+        }
+    }
+}