perf(api,frontend): index hot queries, paginate chats, fix realtime

frontend/src/routes/app/[chatId]/+page.svelte

@@ -11,6 +11,7 @@
     if (accounts.selectedId === null) {
       return;
     }
+    chats.ensure(chatId);
     chats.enrich(chatId);
   });
 </script>