fix: invalidate stale service records PDF cache on service record changes

Ensure admin/API deletes and updates of service records, photos, and
attachments invalidate the cached service-records PDF. Also harden cached
PDF lookup/download paths so a missing file is treated as stale cache
instead of a 500.

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

backend/modules/fleet/views.py

@@ -532,10 +532,30 @@ def _get_cached_pdf(work_order, pdf_type):
     )
     for candidate in candidates:
         if str(candidate.filename or '').strip() == expected_filename:
-            return candidate
+            file_name = getattr(candidate.file, 'name', '')
+            storage = getattr(candidate.file, 'storage', None)
+            if file_name and storage:
+                try:
+                    if storage.exists(file_name):
+                        return candidate
+                except (FileNotFoundError, OSError, ValueError):
+                    pass
+            candidate.file = None
+            candidate.is_active = False
+            candidate.status = 'failed'
+            candidate.error_message = 'PDF datoteka nije dostupna.'
+            candidate.save(update_fields=['file', 'is_active', 'status', 'error_message', 'updated_at'])
     return None
 
 
+def _mark_generated_pdf_failed(generated_pdf, error_message):
+    generated_pdf.file = None
+    generated_pdf.is_active = False
+    generated_pdf.status = 'failed'
+    generated_pdf.error_message = error_message
+    generated_pdf.save(update_fields=['file', 'is_active', 'status', 'error_message', 'updated_at'])
+
+
 def _cleanup_expired_generated_pdfs():
     now = timezone.now()
     expired = GeneratedWorkOrderPdf.objects.filter(
@@ -546,10 +566,11 @@ def _cleanup_expired_generated_pdfs():
     for item in expired:
         if item.file:
             item.file.delete(save=False)
+        item.file = None
         item.is_active = False
         item.status = 'failed'
         item.error_message = 'PDF cache istekao.'
-        item.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+        item.save(update_fields=['file', 'is_active', 'status', 'error_message', 'updated_at'])
 
 
 def _parse_amount_decimal(value):
@@ -572,10 +593,11 @@ def _invalidate_work_order_pdf_cache(work_order, *, pdf_types=None):
     for cached in cache_qs:
         if cached.file:
             cached.file.delete(save=False)
+        cached.file = None
         cached.is_active = False
         cached.status = 'failed'
         cached.error_message = 'PDF cache invalidiran zbog promjene podataka.'
-        cached.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+        cached.save(update_fields=['file', 'is_active', 'status', 'error_message', 'updated_at'])
 
 
 def _upsert_additional_cost_row_from_invoice(invoice):
@@ -607,7 +629,11 @@ def _upsert_additional_cost_row_from_invoice(invoice):
 
 
 def _cached_pdf_file_response(generated_pdf, *, default_filename):
-    generated_pdf.file.open('rb')
+    try:
+        generated_pdf.file.open('rb')
+    except (FileNotFoundError, OSError, ValueError):
+        _mark_generated_pdf_failed(generated_pdf, 'PDF datoteka nije dostupna.')
+        raise DRFValidationError({"detail": "PDF nije dostupan ili je obrisan."})
     filename = generated_pdf.filename or default_filename
     response = FileResponse(generated_pdf.file, content_type='application/pdf')
     response['Content-Disposition'] = f'attachment; filename="{filename}"'
@@ -3666,6 +3692,18 @@ class WorkOrderViewSet(viewsets.ModelViewSet):
         ).exclude(file='').exclude(file__isnull=True).first()
         if generated_pdf is None:
             raise DRFValidationError({"detail": "PDF nije dostupan ili je istekao."})
+        file_name = getattr(generated_pdf.file, 'name', '')
+        storage = getattr(generated_pdf.file, 'storage', None)
+        if not file_name or storage is None:
+            _mark_generated_pdf_failed(generated_pdf, 'PDF datoteka nije dostupna.')
+            raise DRFValidationError({"detail": "PDF nije dostupan ili je obrisan."})
+        try:
+            if not storage.exists(file_name):
+                _mark_generated_pdf_failed(generated_pdf, 'PDF datoteka nije dostupna.')
+                raise DRFValidationError({"detail": "PDF nije dostupan ili je obrisan."})
+        except (FileNotFoundError, OSError, ValueError):
+            _mark_generated_pdf_failed(generated_pdf, 'PDF datoteka nije dostupna.')
+            raise DRFValidationError({"detail": "PDF nije dostupan ili je obrisan."})
         return _cached_pdf_file_response(generated_pdf, default_filename=_pdf_filename(work_order, generated_pdf.pdf_type))
 
     @action(detail=True, methods=['get'], url_path='pdf-preview')