fix: regenerate SN archive with task-specific DOCX headers

Invalidate cached monthly service-task ZIP archives so exports are rebuilt with the latest task/work-order/vehicle header data. Keep stale detection for work-order archives and add regression tests for per-task header isolation and stale cache replacement.

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

backend/modules/fleet/views.py

@@ -23,7 +23,7 @@ from django.http import HttpResponse, FileResponse
 from django.urls import reverse
 from django.utils import timezone
 from django.utils.html import escape
-from django.db.models import Q
+from django.db.models import Q, Max
 from django.db import transaction
 from reportlab.lib.pagesizes import A4
 from reportlab.lib import colors
@@ -2959,6 +2959,33 @@ def _get_cached_generated_archive(*, user, archive_type, year, month):
     )
 
 
+def _latest_monthly_archive_source_update(*, user, archive_type, year, month):
+    from modules.task_management.models import Task
+
+    base_tasks = Task.objects.filter(
+        assigned_to=user,
+        is_active=True,
+        scheduled_date__year=year,
+        scheduled_date__month=month,
+        work_order__isnull=False,
+        work_order__is_active=True,
+    )
+    latest_candidates = [
+        base_tasks.aggregate(value=Max('updated_at')).get('value'),
+        base_tasks.aggregate(value=Max('work_order__updated_at')).get('value'),
+        base_tasks.aggregate(value=Max('work_order__vehicle__updated_at')).get('value'),
+        base_tasks.aggregate(value=Max('work_hours_table__updated_at')).get('value'),
+    ]
+    if archive_type == 'work_orders':
+        latest_candidates.append(
+            base_tasks.aggregate(value=Max('work_order__invoices__updated_at')).get('value')
+        )
+    latest_values = [value for value in latest_candidates if value is not None]
+    if not latest_values:
+        return None
+    return max(latest_values)
+
+
 def _request_monthly_archive_generation(*, request, archive_type):
     year, month = _parse_year_month_params(request)
     _cleanup_expired_generated_archive_records()
@@ -2970,21 +2997,39 @@ def _request_monthly_archive_generation(*, request, archive_type):
         month=month,
     )
     if cached:
-        _notify_monthly_archive_request(
-            user=request.user,
-            archive_type=archive_type,
-            stage='completed',
-            year=year,
-            month=month,
-            generated_archive=cached,
-        )
-        return {
-            'status': 'ready',
-            'generated_archive_id': str(cached.pk),
-            'download_url': f"fleet/reports/generated-archives/{cached.pk}/download/",
-            'filename': cached.filename,
-            'expires_at': cached.expires_at.isoformat() if cached.expires_at else None,
-        }
+        if archive_type == 'service_tasks':
+            cached.is_active = False
+            cached.status = 'failed'
+            cached.error_message = 'ZIP arhiva servisnih taskova se regenerira za najnoviji kontekst.'
+            cached.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+        else:
+            latest_source_update = _latest_monthly_archive_source_update(
+                user=request.user,
+                archive_type=archive_type,
+                year=year,
+                month=month,
+            )
+            if latest_source_update and cached.updated_at and cached.updated_at < latest_source_update:
+                cached.is_active = False
+                cached.status = 'failed'
+                cached.error_message = 'ZIP arhiva zastarjela zbog novijih izmjena izvora.'
+                cached.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+            else:
+                _notify_monthly_archive_request(
+                    user=request.user,
+                    archive_type=archive_type,
+                    stage='completed',
+                    year=year,
+                    month=month,
+                    generated_archive=cached,
+                )
+                return {
+                    'status': 'ready',
+                    'generated_archive_id': str(cached.pk),
+                    'download_url': f"fleet/reports/generated-archives/{cached.pk}/download/",
+                    'filename': cached.filename,
+                    'expires_at': cached.expires_at.isoformat() if cached.expires_at else None,
+                }
 
     existing_pending = (
         GeneratedFleetArchive.objects