fix(m2-m3): data-testid on new-client+add-client btns; fix client subscriptions API response shape; update E2E specs

app/(app)/clients/page.tsx

@@ -90,7 +90,7 @@ export default function ClientsPage() {
         </div>
         <div className="flex gap-2">
           <Button onClick={() => refetch()} variant="outline" size="sm"><RefreshCw size={14} className="mr-1" />Refresh</Button>
-          <Button onClick={() => setShowAdd(true)} size="sm"><UserPlus size={14} className="mr-1" />Add Client</Button>
+          <Button onClick={() => setShowAdd(true)} size="sm" data-testid="add-client-btn"><UserPlus size={14} className="mr-1" />Add Client</Button>
         </div>
       </div>