Require auth for /preview_email/ (#16) (#17)

## Summary
- Closes #16
- Confirms `/preview_email/<pk>/` requires an authenticated user (`@login_required` already on the view)
- Adds unit tests for unauthenticated redirect and authenticated preview access

## Test plan
- [x] `python manage.py test public.tests.PreviewEmailAuthTests`
- [ ] Manually hit `/preview_email/1/` logged out → redirect to `/accounts/login/`
- [ ] Log in and hit same URL → email preview renders

Reviewed-on: #17

company_site/public/tests.py

@@ -1,12 +1,40 @@
 from unittest.mock import patch
 
+from django.contrib.auth.models import User
 from django.test import Client, TestCase, override_settings
 from django.urls import reverse
 
-from .models import Contact
+from .models import Contact, EmailMessage
 from .seo import SERVICE_URL_NAMES, get_service_entries
 
 
+class PreviewEmailAuthTests(TestCase):
+    def setUp(self):
+        self.client = Client()
+        self.user = User.objects.create_user(username="previewer", password="pass")
+        self.email = EmailMessage.objects.create(
+            subject="Preview subject",
+            body="Preview body content",
+            recipient="recipient@example.com",
+        )
+        self.url = reverse("preview_email", kwargs={"pk": self.email.pk})
+
+    def test_unauthenticated_user_is_redirected_to_login(self):
+        response = self.client.get(self.url)
+
+        self.assertEqual(response.status_code, 302)
+        self.assertIn("/accounts/login/", response.url)
+
+    def test_authenticated_user_can_preview_email(self):
+        self.client.login(username="previewer", password="pass")
+
+        response = self.client.get(self.url)
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Preview subject")
+        self.assertContains(response, "Preview body content")
+
+
 @override_settings(
     DEBUG=True,
     EMAIL_BACKEND="django.core.mail.backends.locmem.EmailBackend",