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/admin.py

@@ -1,3 +1,4 @@
+from pathlib import Path
 from django.contrib import admin
 from django.contrib.admin import ModelAdmin
 from django.utils.html import format_html
@@ -61,8 +62,27 @@ class ServiceRecordInline(admin.TabularInline):
 class WorkOrderInvoiceInline(admin.TabularInline):
     model = WorkOrderInvoice
     extra = 0
-    fields = ('naziv_racuna', 'lokacija', 'datum', 'opis', 'image', 'created_by', 'created_at', 'is_active')
-    readonly_fields = ('created_at',)
+    fields = ('thumbnail_preview', 'naziv_racuna', 'lokacija', 'datum', 'opis', 'image', 'created_by', 'created_at', 'is_active')
+    readonly_fields = ('thumbnail_preview', 'created_at')
+
+    @admin.display(description="Pregled")
+    def thumbnail_preview(self, obj):
+        if not obj.pk or not obj.image:
+            return "—"
+        suffix = Path(obj.image.name or '').suffix.lower()
+        if suffix == '.pdf':
+            return format_html(
+                '<a href="/api/fleet/work-order-invoices/{}/image/" target="_blank">📄 PDF</a>',
+                obj.pk,
+            )
+        return format_html(
+            '<a href="/api/fleet/work-order-invoices/{}/image/?w=1600&q=90&fmt=jpeg" target="_blank">'
+            '<img src="/api/fleet/work-order-invoices/{}/image/?w=200&q=75&fmt=jpeg" '
+            'style="height:64px;width:auto;border-radius:4px;object-fit:cover;" />'
+            '</a>',
+            obj.pk,
+            obj.pk,
+        )
 
 
 class WorkOrderPhotoInline(admin.TabularInline):
@@ -193,10 +213,30 @@ class WorkOrderAdmin(admin.ModelAdmin):
 
 @admin.register(WorkOrderInvoice)
 class WorkOrderInvoiceAdmin(admin.ModelAdmin):
-    list_display = ('id', 'work_order', 'naziv_racuna', 'lokacija', 'datum', 'created_by', 'is_active')
+    list_display = ('id', 'image_preview', 'work_order', 'naziv_racuna', 'lokacija', 'datum', 'created_by', 'is_active')
     list_filter = ('datum', 'is_active', 'work_order')
     search_fields = ('naziv_racuna', 'lokacija', 'opis', 'work_order__id')
     ordering = ('-datum', '-created_at')
+    readonly_fields = ('image_preview', 'created_at')
+
+    @admin.display(description="Slika")
+    def image_preview(self, obj):
+        if not obj.pk or not obj.image:
+            return "—"
+        suffix = Path(obj.image.name or '').suffix.lower()
+        if suffix == '.pdf':
+            return format_html(
+                '<a href="/api/fleet/work-order-invoices/{}/image/" target="_blank">📄 PDF</a>',
+                obj.pk,
+            )
+        return format_html(
+            '<a href="/api/fleet/work-order-invoices/{}/image/?w=1600&q=90&fmt=jpeg" target="_blank">'
+            '<img src="/api/fleet/work-order-invoices/{}/image/?w=200&q=75&fmt=jpeg" '
+            'style="height:80px;width:auto;border-radius:4px;object-fit:cover;" />'
+            '</a>',
+            obj.pk,
+            obj.pk,
+        )
 
 
 @admin.register(WorkOrderAdditionalCostsTable)