fix: ispravi prikaz računa u admin/frontendu i smanji PDF slike

- admin.py: thumbnail_preview u WorkOrderInvoiceInline i WorkOrderInvoiceAdmin
  prikazuje slike/PDF veze putem optimiziranog image endpointa
- serializers.py: image_url sada uključuje ?w=1280&q=80&fmt=jpeg za ne-PDF slike
- views.py: doda _compress_image_for_pdf() helper, zamijeni ImageReader(photo.image)
  s kompresiranom JPEG verzijom na oba mjesta u PDF generiranju
- tasks.py: kompresija invoice slike u JPEG u memoriji prije ImageReader poziva
- WorkOrderInvoicesPdfPage.jsx:
  * InvoiceImagePreview prikazuje 'Učitavanje...' dok se blob dohvaća (ne 401 fallback)
  * uklonjen resolveMediaUrl fallback koji uzrokuje 401 greške
  * svaka invoice kartica je klikabilna i otvara InvoiceDetailModal
  * InvoiceDetailModal prikazuje detalje i uvećanu sliku/PDF vezu

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

backend/modules/fleet/serializers.py

@@ -373,9 +373,13 @@ class WorkOrderInvoiceSerializer(serializers.ModelSerializer):
             return None
         if request is None:
             return None
-        return request.build_absolute_uri(
+        base = request.build_absolute_uri(
             reverse('work-order-invoice-image', kwargs={'pk': obj.pk})
         )
+        suffix = Path(obj.image.name or '').suffix.lower()
+        if suffix == '.pdf':
+            return base
+        return f"{base}?w=1280&q=80&fmt=jpeg"
 
     def get_image_content_type(self, obj):
         if not obj.image: