Add extensive controller and model tests

tests/Feature/PreviewControllerTest.php

@@ -0,0 +1,23 @@
+<?php
+
+it('renders bbcode preview', function (): void {
+    $user = \App\Models\User::factory()->create();
+    \Laravel\Sanctum\Sanctum::actingAs($user);
+
+    $response = $this->postJson('/api/preview', [
+        'body' => '[b]Hello[/b]',
+    ]);
+
+    $response->assertOk();
+    $response->assertJsonStructure(['html']);
+});
+
+it('validates preview body', function (): void {
+    $user = \App\Models\User::factory()->create();
+    \Laravel\Sanctum\Sanctum::actingAs($user);
+
+    $response = $this->postJson('/api/preview', []);
+
+    $response->assertStatus(422);
+    $response->assertJsonValidationErrors(['body']);
+});