feat: dodaj prefill email potpisa i body logging

Uvodi signature polje i default predlozak potpisa za nove korisnike, te data migraciju koja popunjava postojece prazne potpise. Email flow sada dosljedno dodaje korisnicki potpis i sprema finalni body poruke u dispatch log radi audita.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/fleet/tests/test_work_order_email_task_cached_pdfs.py

@@ -1,6 +1,7 @@
 from unittest.mock import patch
 
 from django.contrib.auth import get_user_model
+from django.core import mail
 from django.test import TestCase, override_settings
 
 from modules.fleet.models import GeneratedWorkOrderPdf, Vehicle, VehicleNotification, WorkOrder
@@ -58,3 +59,25 @@ class WorkOrderEmailTaskCachedPdfTests(TestCase):
         cached_pdfs = notif.metadata.get('cached_pdfs') if isinstance(notif.metadata, dict) else None
         self.assertTrue(isinstance(cached_pdfs, list) and len(cached_pdfs) > 0)
         self.assertIn('download_url', cached_pdfs[0])
+
+    def test_success_email_appends_requester_signature(self):
+        mail.outbox.clear()
+
+        result = send_work_order_email_bundle_task(
+            work_order_id=str(self.work_order.pk),
+            requested_by_id=str(self.user.pk),
+            recipients=['client@example.test'],
+            message='Test poruka bez potpisa.',
+            include_work_order_pdf=True,
+            include_service_records_pdf=False,
+            include_invoices_pdf=False,
+            include_images=False,
+            include_monthly_tasks=False,
+        )
+
+        self.assertEqual(result.get('status'), 'ok')
+        self.assertEqual(len(mail.outbox), 1)
+        sent_body = mail.outbox[0].body
+        self.assertIn('Test poruka bez potpisa.', sent_body)
+        self.assertIn('S poštovanjem / Freundliche Grüße / Kind regards', sent_body)
+        self.assertIn(self.user.email, sent_body)