feat(scale): swappable pub/sub layer + fix chat.js missed awaits (Phase 6)

Two things:

1. FIX a live regression the async conversion missed: chat.js calls repos via the
   lazy repos() helper (not the R. prefix), so my sweep skipped it — effectiveStatus
   / broadcastPresence read `repos().users.byId(userId)` synchronously, but that's a
   Promise now, so presence broadcasts always reported status 'active' and dropped
   last_seen. Now awaited (effectiveStatus/broadcastPresence async); touchSeen is a
   fire-and-forget UPDATE with .catch. Audited all non-R. repo calls — only chat.js
   was affected (media.js backfill was already awaited).

2. Swappable pub/sub for multi-instance real-time fan-out (the actual blocker to
   running >1 instance — not the DB). server/pubsub.js picks a backend by
   PUBSUB_BACKEND (default 'memory'). Local socket delivery is UNCHANGED; publish is
   additive — memory = no-op (zero hot-path cost, identical single-instance
   behaviour), redis = fan-out to other instances with a self-echo guard. chat.js
   pushToUser/broadcastPresence now also publish; each instance subscribes to deliver
   remote events to its local sockets. Interface is tiny so Redis is one swappable
   file (Postgres LISTEN/NOTIFY or NATS could drop in the same way — never hardwired,
   as requested). Dormant redis service added to compose behind the 'scale' profile;
   redis dep added; PUBSUB_BACKEND/REDIS_URL documented.

Validated: smoke 22/22 (memory), e2e chat delivery green. NOTE: full multi-instance
also needs distributed presence (isOnline is per-process) + meeting-signaling
sharing — chat/presence fan out via this layer; those are follow-ups.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/pubsub/redis.js

@@ -0,0 +1,46 @@
+// Redis pub/sub backend — enables running MULTIPLE app instances. Each instance publishes every local
+// real-time event; every other instance receives it and delivers to ITS local sockets. Selected by
+// PUBSUB_BACKEND=redis; connection from REDIS_URL (default redis://bizgaze-redis:6379).
+//
+// Self-echo guard: Redis delivers a publish to ALL subscribers including the publisher, but the publishing
+// instance ALREADY delivered locally — so every message is tagged with this instance's id and ignored on
+// the way back in. Subscriptions made before connect are buffered and flushed in init().
+const crypto = require('crypto');
+const INSTANCE = crypto.randomBytes(8).toString('hex');
+
+let pub = null, sub = null;
+const pending = []; // [pattern, handler] queued before connect
+
+async function doSubscribe(pattern, handler) {
+  const onMessage = (message, channel) => {
+    let m; try { m = JSON.parse(message); } catch { return; }
+    if (m.i === INSTANCE) return;                 // our own publish — already delivered locally
+    try { handler(channel, m.d); } catch (_) {}
+  };
+  if (pattern.includes('*')) await sub.pSubscribe(pattern, onMessage);
+  else await sub.subscribe(pattern, onMessage);
+}
+
+async function init() {
+  const { createClient } = require('redis');
+  const url = process.env.REDIS_URL || 'redis://bizgaze-redis:6379';
+  pub = createClient({ url });
+  sub = pub.duplicate();
+  pub.on('error', () => {}); sub.on('error', () => {}); // never let a redis blip crash the app
+  await pub.connect();
+  await sub.connect();
+  for (const [pattern, handler] of pending) { try { await doSubscribe(pattern, handler); } catch (_) {} }
+  pending.length = 0;
+}
+
+function publish(channel, data) {
+  if (!pub) return;
+  pub.publish(channel, JSON.stringify({ i: INSTANCE, d: data })).catch(() => {});
+}
+
+function subscribe(pattern, handler) {
+  if (!sub) { pending.push([pattern, handler]); return; } // buffer until init() connects
+  doSubscribe(pattern, handler).catch(() => {});
+}
+
+module.exports = { name: 'redis', init, publish, subscribe };