feat(iios): socket.io Redis adapter for cross-replica realtime (P9 scaling)

Adds an opt-in RedisIoAdapter (wired in main.ts when REDIS_URL is set) so
socket.io room emits fan out across instances via Redis pub/sub — a client on
replica B now receives messages emitted by replica A. Without REDIS_URL the
in-memory adapter is kept (single-instance dev unchanged). Adds redis to
docker-compose, REDIS_URL to the env contract, and smoke-realtime-cluster.mjs
which proves cross-instance delivery fails without Redis and passes with it.
Delivery is at-least-once at N>1; clients dedupe by message id (documented).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/.env.example

@@ -8,6 +8,9 @@
 DATABASE_URL=postgresql://iios:iios@localhost:5434/iios?schema=public   # 🔒 Postgres connection
 PORT=3200
 # NODE_ENV=production   # set by the Docker image
+# Realtime fan-out across replicas (socket.io Redis adapter). REQUIRED when running
+# more than one instance with live chat; omit for a single instance (in-memory adapter).
+# REDIS_URL=redis://localhost:6379                                       # 🔒
 
 # ── Secrets ──────────────────────────────────────────────────────────────────
 # JSON map of appId → HS256 signing secret used to verify session JWTs (SessionVerifier).