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/models.py

@@ -2,10 +2,12 @@ import uuid
 import re
 from datetime import time
 from django.db import models
+from django.db.models.signals import post_save, pre_delete
 from django.conf import settings
 from django.core.exceptions import ValidationError
 from django.utils.translation import gettext_lazy as _
 from django.utils import timezone
+from django.dispatch import receiver
 from core.base_models import BaseModel
 from decimal import Decimal
 
@@ -142,6 +144,67 @@ class VehicleServiceAttachment(BaseModel):
     def __str__(self):
         return f"Attachment {self.pk} for {self.service_record} ({self.file.name if self.file else 'no-file'})"
 
+
+def _service_record_work_order_id(instance):
+    if isinstance(instance, VehicleServiceRecord):
+        task = getattr(instance, 'task', None)
+        if task and getattr(task, 'work_order_id', None):
+            return task.work_order_id
+        if getattr(instance, 'task_id', None):
+            return VehicleServiceRecord.objects.filter(pk=instance.pk).values_list('task__work_order_id', flat=True).first()
+        return None
+
+    service_record = getattr(instance, 'service_record', None)
+    if service_record and getattr(service_record, 'task', None) and getattr(service_record.task, 'work_order_id', None):
+        return service_record.task.work_order_id
+
+    service_record_id = getattr(instance, 'service_record_id', None)
+    if not service_record_id:
+        return None
+    return VehicleServiceRecord.objects.filter(pk=service_record_id).values_list('task__work_order_id', flat=True).first()
+
+
+def _invalidate_service_records_pdf_cache(work_order_id):
+    if not work_order_id:
+        return
+
+    cached_pdfs = GeneratedWorkOrderPdf.objects.filter(
+        is_active=True,
+        work_order_id=work_order_id,
+        pdf_type='service_records',
+    )
+    for cached in cached_pdfs:
+        file_name = getattr(cached.file, 'name', '')
+        if file_name:
+            try:
+                if cached.file.storage.exists(file_name):
+                    cached.file.delete(save=False)
+            except (FileNotFoundError, OSError, ValueError):
+                pass
+        cached.file = None
+        cached.is_active = False
+        cached.status = 'failed'
+        cached.error_message = 'PDF cache invalidiran zbog promjene servisnih zapisa.'
+        cached.save(update_fields=['file', 'is_active', 'status', 'error_message', 'updated_at'])
+
+
+@receiver(post_save, sender='fleet.VehicleServiceRecord')
+@receiver(pre_delete, sender='fleet.VehicleServiceRecord')
+def _invalidate_service_records_pdf_for_service_record(sender, instance, **kwargs):
+    _invalidate_service_records_pdf_cache(_service_record_work_order_id(instance))
+
+
+@receiver(post_save, sender='fleet.VehicleServicePhoto')
+@receiver(pre_delete, sender='fleet.VehicleServicePhoto')
+def _invalidate_service_records_pdf_for_service_photo(sender, instance, **kwargs):
+    _invalidate_service_records_pdf_cache(_service_record_work_order_id(instance))
+
+
+@receiver(post_save, sender='fleet.VehicleServiceAttachment')
+@receiver(pre_delete, sender='fleet.VehicleServiceAttachment')
+def _invalidate_service_records_pdf_for_service_attachment(sender, instance, **kwargs):
+    _invalidate_service_records_pdf_cache(_service_record_work_order_id(instance))
+
 class Vehicle(BaseModel):
     ASSET_TYPE_CHOICES = [
         ('vehicle', _("Vozilo")),