feat: dodaj display_code polje za putne naloge (MT070626 format)

- Dodano polje 'display_code' na WorkOrder model (format: 2 inicijala + ddmmyy)
- Migracije za kreiranje polja i backfill postojećih naloga
- Ažuriran WorkOrderSerializer za validaciju i serijalizaciju display_code
- Backend tasks za auto-generiranje display_code pri kreiranju
- Frontend komponente ažurirane za prikaz display_code umjesto UUID:
  * FleetDashboardShell tablica putnih naloga
  * WorkOrderDetailModal
  * TaskCreateModal
  * Sve PDF stranice
- displayIds.js utility za formatiranjeDisplay ID-eva
- Admin interface prikazuje display_code

backend/modules/fleet/views.py

@@ -142,17 +142,26 @@ def _parse_format(value):
 GENERATED_PDF_TTL_HOURS = 24
 
 
+def _work_order_display_code(work_order):
+    normalized = str(getattr(work_order, 'display_code', '') or '').strip().upper()
+    if normalized:
+        return normalized
+    return 'NALOG'
+
+
 def _pdf_filename(work_order, pdf_type):
+    display_code = _work_order_display_code(work_order)
     if pdf_type == 'invoices':
-        return f"{work_order.pk}.work-order-invoices.pdf"
+        return f"{display_code}.work-order-invoices.pdf"
     if pdf_type == 'service_records':
-        return f"{work_order.pk}.work-order-service-records.pdf"
-    return f"{work_order.pk}.work-order.pdf"
+        return f"{display_code}.work-order-service-records.pdf"
+    return f"{display_code}.work-order.pdf"
 
 
 def _get_cached_pdf(work_order, pdf_type):
     now = timezone.now()
-    return (
+    expected_filename = _pdf_filename(work_order, pdf_type)
+    candidates = list(
         GeneratedWorkOrderPdf.objects
         .filter(
             is_active=True,
@@ -164,8 +173,11 @@ def _get_cached_pdf(work_order, pdf_type):
         .exclude(file='')
         .exclude(file__isnull=True)
         .order_by('-created_at')
-        .first()
     )
+    for candidate in candidates:
+        if str(candidate.filename or '').strip() == expected_filename:
+            return candidate
+    return None
 
 
 def _cleanup_expired_generated_pdfs():
@@ -203,17 +215,18 @@ def _notify_pdf_request(*, user, work_order, doc_type, stage, generated_pdf=None
     else:
         doc_label = "računa putnog naloga"
 
+    display_code = _work_order_display_code(work_order)
     if stage == 'requested':
         title = f"Zahtjev za PDF {doc_label}"
-        message = f"Zaprimljen je zahtjev za generiranje PDF dokumenta za WO-{work_order.pk}."
+        message = f"Zaprimljen je zahtjev za generiranje PDF dokumenta za putni nalog {display_code}."
         level = "info"
     elif stage == 'failed':
         title = f"Greška kod PDF-a ({doc_label})"
-        message = f"Generiranje PDF dokumenta nije uspjelo za WO-{work_order.pk}."
+        message = f"Generiranje PDF dokumenta nije uspjelo za putni nalog {display_code}."
         level = "warning"
     else:
         title = f"PDF spreman ({doc_label})"
-        message = f"PDF dokument je uspješno generiran za WO-{work_order.pk}."
+        message = f"PDF dokument je uspješno generiran za putni nalog {display_code}."
         level = "success"
 
     metadata = {
@@ -337,6 +350,7 @@ def _build_work_order_pdf(work_order):
     invoices = list(work_order.invoices.filter(is_active=True).order_by('-datum', '-created_at'))
     now_local = timezone.localtime(timezone.now())
     tz = timezone.get_current_timezone()
+    display_code = _work_order_display_code(work_order)
 
     def _fmt_date(value):
         if not value:
@@ -437,7 +451,7 @@ def _build_work_order_pdf(work_order):
         _paragraph("N A L O G<br/>ZA SLUŽBENO PUTOVANJE", bold=True, size=11, align=1),
         _paragraph(
             f"Mjesto i datum:<br/><b>{place_label} &nbsp;&nbsp; {_fmt_date(now_local.date())}</b><br/>"
-            f"Broj naloga:<br/><b>{work_order.pk}</b>",
+            f"Broj naloga:<br/><b>{display_code}</b>",
             size=8.8,
         ),
     ]]
@@ -686,7 +700,7 @@ def _build_work_order_service_records_pdf(work_order):
     upgrade_hours_str = str(getattr(vehicle, 'superstructure_working_hours', '-') or '-')
     chassis_hours_str = str(getattr(vehicle, 'chassis_working_hours', '-') or '-')
     mileage_str = str(vehicle.current_mileage or '-')
-    nalog_str = str(work_order.pk)
+    nalog_str = _work_order_display_code(work_order)
     generated_date_str = timezone.localtime(timezone.now()).strftime('%d.%m.%Y')
 
     HEADER_H = 86
@@ -1064,7 +1078,7 @@ def _build_service_record_pdf(service_record):
     upgrade_hours_str = str(getattr(vehicle, 'superstructure_working_hours', '-') or '-')
     chassis_hours_str = str(getattr(vehicle, 'chassis_working_hours', '-') or '-')
     mileage_str = str(vehicle.current_mileage or '-')
-    nalog_str = str(getattr(work_order, 'pk', '-') or '-')
+    nalog_str = _work_order_display_code(work_order) if work_order else '-'
     generated_date_str = timezone.localtime(timezone.now()).strftime('%d.%m.%Y')
 
     HEADER_H = 86
@@ -1487,7 +1501,7 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         pdf_bytes = _build_work_order_pdf(work_order)
         _notify_pdf_request(user=request.user, work_order=work_order, doc_type='work_order', stage='completed')
         response = HttpResponse(pdf_bytes, content_type='application/pdf')
-        response['Content-Disposition'] = f'attachment; filename="{work_order.pk}.work-order.pdf"'
+        response['Content-Disposition'] = f'attachment; filename="{_pdf_filename(work_order, "work_order")}"'
         return response
 
     @action(detail=True, methods=['post'], url_path='pdf-request')
@@ -1508,7 +1522,7 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         pdf_bytes = _build_work_order_service_records_pdf(work_order)
         _notify_pdf_request(user=request.user, work_order=work_order, doc_type='service_records', stage='completed')
         response = HttpResponse(pdf_bytes, content_type='application/pdf')
-        response['Content-Disposition'] = f'attachment; filename="{work_order.pk}.work-order-service-records.pdf"'
+        response['Content-Disposition'] = f'attachment; filename="{_pdf_filename(work_order, "service_records")}"'
         return response
 
     @action(detail=True, methods=['post'], url_path='service-records-pdf-request')
@@ -1545,7 +1559,7 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         except (ValueError, TypeError):
             raise DRFValidationError({"detail": "Neispravan PDF sadržaj računa."})
 
-        filename = payload.get('filename') or f"{work_order.pk}.work-order-invoices.pdf"
+        filename = payload.get('filename') or _pdf_filename(work_order, 'invoices')
         _notify_pdf_request(user=request.user, work_order=work_order, doc_type='invoices', stage='completed')
         response = HttpResponse(pdf_bytes, content_type='application/pdf')
         response['Content-Disposition'] = f'attachment; filename="{filename}"'
@@ -1600,7 +1614,7 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         <head>
             <meta charset="utf-8" />
             <meta name="viewport" content="width=device-width, initial-scale=1" />
-            <title>Računi putnog naloga WO-{work_order.pk}</title>
+            <title>Računi putnog naloga {_work_order_display_code(work_order)}</title>
             <style>
                 body {{ font-family: Arial, sans-serif; margin: 24px; color: #111827; background: #f8fafc; }}
                 .card {{ background: #fff; border: 1px solid #e5e7eb; border-radius: 12px; padding: 16px; margin-bottom: 16px; }}
@@ -1614,7 +1628,7 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         </head>
         <body>
             <div class="card">
-                <h1>Putni nalog WO-{work_order.pk} — Računi</h1>
+                <h1>Putni nalog {_work_order_display_code(work_order)} — Računi</h1>
                 <p class="muted">Dizalica: {escape(work_order.vehicle.registration_number)} | Datum naloga: {escape(str(work_order.date))}</p>
                 <div class="actions">
                     <a href="{escape(pdf_url)}">Preuzmi PDF putnog naloga</a>
@@ -1693,13 +1707,13 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
             raise DRFValidationError({"recipient": "Nije pronađena email adresa za slanje."})
 
         pdf_bytes = _build_work_order_pdf(work_order)
-        subject = _first_non_empty(request.data.get('subject')) or f"Putni nalog WO-{work_order.pk}"
+        subject = _first_non_empty(request.data.get('subject')) or f"Putni nalog {_work_order_display_code(work_order)}"
         body = _first_non_empty(request.data.get('message')) or "U prilogu je PDF putnog naloga."
         _send_document_email(
             recipient=recipient,
             subject=subject,
             body=body,
-            filename=f"{work_order.pk}.work-order.pdf",
+            filename=_pdf_filename(work_order, 'work_order'),
             pdf_bytes=pdf_bytes,
         )
         return Response({"sent": True, "recipient": recipient}, status=status.HTTP_200_OK)