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/server.js

@@ -40,6 +40,7 @@ wss.on('connection', onConnection);
 // no-op (the schema is already applied synchronously when db.js is required). Serving only starts once the
 // store is ready, so the first request can never hit a missing table.
 const db = require('./dbx');
+const pubsub = require('./pubsub');
 
 function startListening() {
   server.listen(PORT, () => {
@@ -72,6 +73,11 @@ function startListening() {
   }
 }
 
-db.init().then(startListening).catch((e) => { console.error('DB init failed:', (e && e.message) || e); process.exit(1); });
+// DB schema first, then the pub/sub layer (redis connects + flushes buffered subscriptions; memory is a
+// no-op), then serve. A pubsub failure must NOT block booting — degrade to local-only delivery.
+db.init()
+  .then(() => pubsub.init().catch((e) => console.error('pubsub init failed (local-only delivery):', (e && e.message) || e)))
+  .then(startListening)
+  .catch((e) => { console.error('DB init failed:', (e && e.message) || e); process.exit(1); });
 
 module.exports = { server };