feat: async PDF cache i API media putanje za putne naloge

Uvodi background request tok za generiranje PDF-a s 24h cache-om i cleanup taskom preko Celery beat-a. Dodaje notification metadata download_url za ponovno preuzimanje bez regeneracije, API image endpointe za putne naloge i račune, te frontend integraciju za async download i produkcijski prikaz slika.

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

backend/modules/fleet/serializers.py

@@ -1,5 +1,6 @@
 from rest_framework import serializers
 import re
+from django.urls import reverse
 from .models import (
     Crane,
     Vehicle,
@@ -183,6 +184,8 @@ class WorkOrderSerializer(serializers.ModelSerializer):
 
 
 class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
+    image_url = serializers.SerializerMethodField()
+
     class Meta:
         model = WorkOrderInvoice
         fields = [
@@ -193,11 +196,22 @@ class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
             'datum',
             'opis',
             'image',
+            'image_url',
             'created_by',
             'created_at',
         ]
         read_only_fields = ['id', 'created_by', 'created_at']
 
+    def get_image_url(self, obj):
+        request = self.context.get('request')
+        if not obj.image:
+            return None
+        if request is None:
+            return None
+        return request.build_absolute_uri(
+            reverse('work-order-invoice-image', kwargs={'pk': obj.pk})
+        )
+
 
 class WorkOrderPhotoSerializer(serializers.ModelSerializer):
     class Meta: