feat: dodaj ishodište i PDF račune za putni nalog

Dodano je origin_location polje kroz backend model, serializer, admin i UI
modal za kreiranje/uređivanje putnog naloga (default: Zagreb).

Računi putnog naloga sada prihvaćaju slike i PDF datoteke, uz backend
validaciju formata, podršku za serving PDF-a i prilagodbu preview prikaza.

Za greške uploada računa dodan je error toast kako bi poruke validacije bile
vidljive korisniku odmah u sučelju.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/fleet/tests/test_work_order_images_endpoint.py

@@ -22,6 +22,15 @@ def create_test_image(filename='test.jpg', size=(40, 40), color='red'):
     )
 
 
+def create_test_pdf(filename='test.pdf'):
+    content = b'%PDF-1.4\n1 0 obj<</Type/Catalog>>endobj\ntrailer<</Root 1 0 R>>\n%%EOF'
+    return SimpleUploadedFile(
+        name=filename,
+        content=content,
+        content_type='application/pdf',
+    )
+
+
 class WorkOrderImagesEndpointTests(TestCase):
     def setUp(self):
         user_model = get_user_model()
@@ -138,3 +147,23 @@ class WorkOrderImagesEndpointTests(TestCase):
         image_response = self.client.get(image_url)
         self.assertEqual(image_response.status_code, 200)
         self.assertTrue(image_response['Content-Type'].startswith('image/'))
+
+    def test_work_order_invoice_serializer_supports_pdf_file(self):
+        invoice = WorkOrderInvoice.objects.create(
+            work_order=self.work_order,
+            naziv_racuna='PDF račun',
+            image=create_test_pdf('invoice.pdf'),
+            created_by=self.user,
+        )
+        list_url = f"{reverse('work-order-invoice-list')}?work_order_id={self.work_order.pk}"
+        list_response = self.client.get(list_url)
+        self.assertEqual(list_response.status_code, 200)
+        self.assertEqual(len(list_response.data), 1)
+        image_url = list_response.data[0].get('image_url')
+        self.assertIsNotNone(image_url)
+        self.assertIn(f'/api/fleet/work-order-invoices/{invoice.pk}/image/', image_url)
+        self.assertEqual(list_response.data[0].get('image_content_type'), 'application/pdf')
+
+        file_response = self.client.get(image_url)
+        self.assertEqual(file_response.status_code, 200)
+        self.assertEqual(file_response['Content-Type'], 'application/pdf')