fix: dohvat mjesecnih racuna za troskove

Uklanja ovisnost o workOrders.invoices i dohvaća račune direktno kroz API filtriran po mjesecu. Dodaje i display code putnog naloga u serializer kako bi prikaz u izvještaju ostao potpun.

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

backend/modules/fleet/serializers.py

@@ -379,12 +379,14 @@ class WorkOrderSerializer(serializers.ModelSerializer):
 class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
     image_url = serializers.SerializerMethodField()
     image_content_type = serializers.SerializerMethodField()
+    work_order_display_code = serializers.SerializerMethodField()
 
     class Meta:
         model = WorkOrderInvoice
         fields = [
             'id',
             'work_order',
+            'work_order_display_code',
             'naziv_racuna',
             'lokacija',
             'datum',
@@ -427,6 +429,15 @@ class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
             return 'image/gif'
         return 'application/octet-stream'
 
+    def get_work_order_display_code(self, obj):
+        work_order = getattr(obj, 'work_order', None)
+        if work_order is None:
+            return None
+        display_code = str(getattr(work_order, 'display_code', '') or '').strip()
+        if display_code:
+            return display_code
+        return str(getattr(work_order, 'id', '') or '').strip() or None
+
     def validate_image(self, value):
         if not value:
             return value