add real-time infrastructure (#137)

* setup infra for pusher service

* setup client sdk for pusher service

* fix: ping parse failure

* fix: send pong back to client

avoid disconnections every 2.5 minutes

* increase replicas

* feat: show presence and compose indicator

js/src/hooks/use-channel.ts

@@ -0,0 +1,91 @@
+import { useEffect, useState, useCallback, useRef } from "react";
+import { usePusherClient } from "@/lib/pusher-provider";
+import type { ChannelMessage } from "@/lib/pusher-client";
+
+interface UseChannelResult {
+  /** Current set of humanIds present in the channel */
+  presence: string[];
+  /** Messages received on this channel (since the hook mounted) */
+  messages: ChannelMessage[];
+  /** Send a message to the channel */
+  sendMessage: (payload: unknown) => void;
+}
+
+/**
+ * Subscribe to a pusher channel. Manages presence tracking and message delivery.
+ * Subscribes on mount, unsubscribes on unmount.
+ *
+ * @param channelId - The channel to subscribe to, or null to skip.
+ */
+export function useChannel(channelId: string | null): UseChannelResult {
+  const client = usePusherClient();
+  const [presence, setPresence] = useState<string[]>([]);
+  const [messages, setMessages] = useState<ChannelMessage[]>([]);
+
+  // Keep a ref to avoid re-subscribing when sendMessage changes
+  const clientRef = useRef(client);
+  const channelRef = useRef(channelId);
+  clientRef.current = client;
+  channelRef.current = channelId;
+
+  useEffect(() => {
+    if (!client || !channelId) {
+      setPresence([]);
+      setMessages([]);
+      return;
+    }
+
+    client.subscribe(channelId);
+
+    const onSubscribed = (msg: { presence?: string[] }) => {
+      setPresence(msg.presence ?? []);
+    };
+
+    const onJoin = (msg: { humanId?: string }) => {
+      if (msg.humanId) {
+        setPresence((prev) =>
+          prev.includes(msg.humanId!) ? prev : [...prev, msg.humanId!],
+        );
+      }
+    };
+
+    const onLeave = (msg: { humanId?: string }) => {
+      if (msg.humanId) {
+        setPresence((prev) => prev.filter((id) => id !== msg.humanId));
+      }
+    };
+
+    const onMessage = (msg: { humanId?: string; payload?: unknown }) => {
+      if (msg.humanId) {
+        setMessages((prev) => [
+          ...prev,
+          { humanId: msg.humanId!, payload: msg.payload },
+        ]);
+      }
+    };
+
+    client.on(channelId, "subscribed", onSubscribed);
+    client.on(channelId, "join", onJoin);
+    client.on(channelId, "leave", onLeave);
+    client.on(channelId, "message", onMessage);
+
+    return () => {
+      client.off(channelId, "subscribed", onSubscribed);
+      client.off(channelId, "join", onJoin);
+      client.off(channelId, "leave", onLeave);
+      client.off(channelId, "message", onMessage);
+      client.unsubscribe(channelId);
+    };
+  }, [client, channelId]);
+
+  const sendMessage = useCallback(
+    (payload: unknown) => {
+      if (clientRef.current && channelRef.current) {
+        clientRef.current.sendMessage(channelRef.current, payload);
+      }
+    },
+    [],
+  );
+
+  return { presence, messages, sendMessage };
+}