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/serializers.py

@@ -1,6 +1,7 @@
 from rest_framework import serializers
 import re
 from django.urls import reverse
+from django.utils import timezone
 from .models import (
     Crane,
     Vehicle,
@@ -98,7 +99,7 @@ class WorkOrderSerializer(serializers.ModelSerializer):
     class Meta:
         model = WorkOrder
         fields = [
-            'id', 'vehicle', 'crane', 'crane_label', 'creator', 'date',
+            'id', 'display_code', 'vehicle', 'crane', 'crane_label', 'creator', 'date',
             'start_mileage', 'end_mileage',
             'servicer_vehicle_registration', 'servicer_vehicle_make_model',
             'servicer_vehicle_start_mileage', 'servicer_vehicle_end_mileage',
@@ -112,6 +113,7 @@ class WorkOrderSerializer(serializers.ModelSerializer):
             'creator': {'required': False},
             'start_mileage': {'required': False, 'allow_null': True},
             'end_mileage': {'required': False, 'allow_null': True},
+            'display_code': {'required': False, 'allow_blank': False},
             'servicer_vehicle_registration': {'required': False, 'allow_blank': True},
             'servicer_vehicle_make_model': {'required': False, 'allow_blank': True},
             'servicer_vehicle_start_mileage': {'required': False, 'allow_null': True},
@@ -146,6 +148,14 @@ class WorkOrderSerializer(serializers.ModelSerializer):
 
         if user and 'creator' not in validated_data:
             validated_data['creator'] = user
+        if not validated_data.get('display_code'):
+            creator = validated_data.get('creator')
+            generated_display_code = self._generate_display_code(creator, timezone.localdate())
+            if not generated_display_code:
+                raise serializers.ValidationError({
+                    'display_code': 'Serviser mora imati ime i prezime za automatsko generiranje oznake. Unesite oznaku ručno u formatu MT070626.'
+                })
+            validated_data['display_code'] = generated_display_code
 
         return services.VehicleService.create_work_order(validated_data)
 
@@ -182,6 +192,22 @@ class WorkOrderSerializer(serializers.ModelSerializer):
             raise serializers.ValidationError({'servicer_vehicle_fuel_cost': 'Trošak goriva ne može biti negativan.'})
         return attrs
 
+    def validate_display_code(self, value):
+        normalized = str(value or '').strip().upper()
+        if not re.fullmatch(r"[A-Z]{2}[0-9]{6}", normalized):
+            raise serializers.ValidationError('Prikaz putnog naloga mora biti u formatu MT070626 (2 slova + 6 znamenki).')
+        return normalized
+
+    @staticmethod
+    def _generate_display_code(user, target_date):
+        if user is None:
+            return None
+        first_name = str(getattr(user, 'first_name', '') or '').strip()
+        last_name = str(getattr(user, 'last_name', '') or '').strip()
+        if not first_name or not last_name:
+            return None
+        return f"{first_name[0].upper()}{last_name[0].upper()}{target_date.strftime('%d%m%y')}"
+
 
 class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
     image_url = serializers.SerializerMethodField()