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>

docker-compose.yml

@@ -63,6 +63,20 @@ services:
       timeout: 3s
       retries: 12
 
+  # Redis — cross-instance real-time fan-out (chat/presence), used only when PUBSUB_BACKEND=redis. Dormant
+  # by default (behind the 'scale' profile, like livekit), so a normal deploy never starts it and the app
+  # stays single-instance on the in-memory pubsub. To run multiple app instances: start this
+  # (`docker compose --profile scale up -d`), set PUBSUB_BACKEND=redis + REDIS_URL in .env, and put the app
+  # behind a load balancer with sticky sessions for the /ws WebSocket. (Meeting SIGNALING state is still
+  # per-process — cross-instance meetings need sticky routing or further work; chat/presence fan out here.)
+  bizgazeredis:
+    image: redis:7-alpine
+    container_name: bizgaze-redis
+    restart: unless-stopped
+    profiles: ["scale"]
+    networks:
+      - npm
+
   # LiveKit SFU — meeting media server. Optional: only started/used when the app's .env has
   # LIVEKIT_URL/API_KEY/API_SECRET set (otherwise meetings use the built-in P2P mesh). NPM proxies
   # wss://livekit.bizgaze.com -> livekit:7880 (signaling); media flows over the published UDP/TCP