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/admin.py

@@ -44,7 +44,7 @@ class ServiceAttachmentInline(admin.TabularInline):
 class WorkOrderInline(admin.TabularInline):
     model = WorkOrder
     extra = 0
-    fields = ('creator', 'date', 'start_mileage', 'end_mileage', 'location', 'travel_start_at', 'travel_end_at', 'purpose', 'status')
+    fields = ('display_code', 'creator', 'date', 'start_mileage', 'end_mileage', 'location', 'travel_start_at', 'travel_end_at', 'purpose', 'status')
     readonly_fields = ('date',)
 
 
@@ -176,9 +176,9 @@ class CraneAdmin(VehicleAdmin):
 
 @admin.register(WorkOrder)
 class WorkOrderAdmin(admin.ModelAdmin):
-    list_display = ('id', 'vehicle', 'creator', 'date', 'location', 'travel_start_at', 'travel_end_at', 'start_mileage', 'end_mileage', 'distance_display', 'status')
+    list_display = ('display_code', 'id', 'vehicle', 'creator', 'date', 'location', 'travel_start_at', 'travel_end_at', 'start_mileage', 'end_mileage', 'distance_display', 'status')
     list_filter = ('status', 'date', 'vehicle')
-    search_fields = ('vehicle__registration_number', 'creator__username', 'purpose')
+    search_fields = ('display_code', 'vehicle__registration_number', 'creator__username', 'purpose')
     readonly_fields = ('date',)
     ordering = ('-date',)
     inlines = (WorkOrderInvoiceInline, WorkOrderPhotoInline)