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

apps/web/src/api/auditLogs.ts

@@ -0,0 +1,31 @@
+import client from './client';
+import type { PaginatedResponse } from './users';
+
+export interface AuditLogItem {
+  id: string;
+  tenant_id: string;
+  action: string;
+  resource_type: string;
+  resource_id: string;
+  user_id: string;
+  old_value?: string;
+  new_value?: string;
+  ip_address?: string;
+  user_agent?: string;
+  created_at: string;
+}
+
+export interface AuditLogQuery {
+  resource_type?: string;
+  user_id?: string;
+  page?: number;
+  page_size?: number;
+}
+
+export async function listAuditLogs(query: AuditLogQuery = {}) {
+  const { data } = await client.get<{ success: boolean; data: PaginatedResponse<AuditLogItem> }>(
+    '/audit-logs',
+    { params: { page: query.page ?? 1, page_size: query.page_size ?? 20, ...query } },
+  );
+  return data.data;
+}