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/tasks.py

@@ -645,10 +645,19 @@ def _build_work_order_invoices_pdf(work_order):
                     y = content_top
                     max_height = y - content_bottom
 
+                # Kompresiraj u JPEG u memoriji radi manje veličine PDF-a
+                compress_w = min(image.width, 1280)
+                if image.width > compress_w:
+                    ratio_c = compress_w / float(image.width)
+                    image = image.resize((compress_w, max(1, int(image.height * ratio_c))), Image.LANCZOS)
+                buf = BytesIO()
+                image.save(buf, format='JPEG', quality=75, optimize=True)
+                buf.seek(0)
+
                 ratio = min(max_width / float(image.width), max_height / float(image.height), 1.0)
                 draw_width = max(1, int(image.width * ratio))
                 draw_height = max(1, int(image.height * ratio))
-                image_reader = ImageReader(image)
+                image_reader = ImageReader(buf)
                 pdf.drawImage(
                     image_reader,
                     margin,