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/messages.ts

@@ -81,7 +81,7 @@ export async function deleteMessage(id: string) {
 
 export async function sendMessage(req: SendMessageRequest) {
   const { data } = await client.post<{ success: boolean; data: MessageInfo }>(
-    '/messages/send',
+    '/messages',
     req,
   );
   return data.data;