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/views.py

@@ -208,6 +208,32 @@ def _docx_filename(work_order, doc_type):
     return f"{display_code}.work-order.docx"
 
 
+def _compress_image_for_pdf(image_field, max_width=1280, quality=75):
+    """
+    Otvori image_field (Django FileField), kompresiraj na max_width JPEG u memoriji,
+    vrati ImageReader spreman za reportlab. Vraća None ako slika nije dostupna.
+    """
+    try:
+        image_field.open('rb')
+        with Image.open(image_field) as src:
+            img = src.convert('RGB')
+            if img.width > max_width:
+                ratio = max_width / float(img.width)
+                new_h = max(1, int(img.height * ratio))
+                img = img.resize((max_width, new_h), Image.LANCZOS)
+            buf = BytesIO()
+            img.save(buf, format='JPEG', quality=quality, optimize=True)
+            buf.seek(0)
+        return ImageReader(buf)
+    except Exception:
+        return None
+    finally:
+        try:
+            image_field.close()
+        except Exception:
+            pass
+
+
 def _get_cached_pdf(work_order, pdf_type):
     now = timezone.now()
     expected_filename = _pdf_filename(work_order, pdf_type)
@@ -1166,7 +1192,9 @@ def _build_work_order_service_records_pdf(work_order):
                     prepared = []
                     for photo in row_photos:
                         try:
-                            image_reader = ImageReader(photo.image)
+                            image_reader = _compress_image_for_pdf(photo.image)
+                            if image_reader is None:
+                                continue
                             source_w, source_h = image_reader.getSize()
                             if not source_w or not source_h:
                                 continue
@@ -1390,7 +1418,9 @@ def _build_service_record_pdf(service_record):
             prepared = []
             for photo in row_photos:
                 try:
-                    image_reader = ImageReader(photo.image)
+                    image_reader = _compress_image_for_pdf(photo.image)
+                    if image_reader is None:
+                        continue
                     source_w, source_h = image_reader.getSize()
                     if not source_w or not source_h:
                         continue