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>

docker-compose.yml

@@ -16,5 +16,18 @@ services:
       timeout: 5s
       retries: 10
 
+  # Realtime fan-out across replicas (socket.io Redis adapter). Point the service at it
+  # with REDIS_URL=redis://localhost:6379; without REDIS_URL the in-memory adapter is used.
+  redis:
+    image: redis:7-alpine
+    container_name: iios-redis
+    ports:
+      - "6379:6379"
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+
 volumes:
   iios_postgres_data: