fix: recover stale pending monthly ZIP requests

Mark stale pending fleet archives as failed after a timeout window and create a fresh request so users receive completion notifications and can download ZIP files again.

Add regression coverage for replacing stale pending service-task archive requests.

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

backend/modules/fleet/views.py

@@ -3055,10 +3055,17 @@ def _request_monthly_archive_generation(*, request, archive_type):
         .first()
     )
     if existing_pending:
-        return {
-            'status': 'processing',
-            'generated_archive_id': str(existing_pending.pk),
-        }
+        stale_pending_threshold = timezone.now() - timedelta(minutes=3)
+        if existing_pending.created_at and existing_pending.created_at < stale_pending_threshold:
+            existing_pending.is_active = False
+            existing_pending.status = 'failed'
+            existing_pending.error_message = 'ZIP zahtjev je ostao predugo u pending statusu; pokrece se novi zahtjev.'
+            existing_pending.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+        else:
+            return {
+                'status': 'processing',
+                'generated_archive_id': str(existing_pending.pk),
+            }
 
     generated_archive = GeneratedFleetArchive.objects.create(
         requested_by=request.user,