fix: handle PolicyAttachConflict when adding project policies

Global policies conflict with project-level attach - treat as success
since the user already has the permission globally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/apps/monitor/views.py

@@ -610,7 +610,11 @@ def iam_user_project_add_view(request, pk):
                                               d['project_name'])
                 attached.append(policy_name)
             except VolcengineAPIError as e:
-                auth_errors.append(f"{policy_name}: {e}")
+                if 'PolicyAttachConflict' in str(e):
+                    # 全局已有此策略，项目级无需重复附加，视为成功
+                    attached.append(policy_name)
+                else:
+                    auth_errors.append(f"{policy_name}: {e}")
 
         obj.attached_policies = attached
         obj.save(update_fields=['attached_policies'])