fix: resolve E2E audit findings and add Phase C frontend pages

- Fix audit_log handler multi-tenant bug: use Extension<TenantContext>
  instead of hardcoded default_tenant_id
- Fix sendMessage route mismatch: frontend /messages/send → /messages
- Add POST /users/{id}/roles backend route for role assignment
- Add task.completed event payload: started_by + instance_id for
  notification delivery
- Add audit log viewer frontend page (AuditLogViewer.tsx)
- Add language management frontend page (LanguageManager.tsx)
- Add api/auditLogs.ts and api/languages.ts modules

crates/erp-workflow/src/service/task_service.rs

@@ -241,7 +241,12 @@ impl TaskService {
         event_bus.publish(erp_core::events::DomainEvent::new(
             "task.completed",
             tenant_id,
-            serde_json::json!({ "task_id": id, "outcome": req.outcome }),
+            serde_json::json!({
+                "task_id": id,
+                "instance_id": instance_id,
+                "started_by": instance.started_by,
+                "outcome": req.outcome,
+            }),
         ), db).await;
 
         audit_service::record(