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/config/seed.php

@@ -0,0 +1,9 @@
+<?php
+
+return [
+    'admin' => [
+        'email' => env('ADMIN_EMAIL', 'admin@tide.test'),
+        'display_name' => env('ADMIN_DISPLAY_NAME', 'admin'),
+        'password' => env('ADMIN_PASSWORD', 'changemenow'),
+    ],
+];