feat(presence): Redis-backed presence for multi-replica prod

Extract a PresencePort seam (async setFocus/clearSocket/isViewing) with two
impls: the existing in-memory Map (dev, single instance) and a new
RedisPresenceService (prod). MessageModule provides PRESENCE_PORT via a
REDIS_URL-gated factory — same pattern as the socket.io Redis adapter — so
'who is viewing what' is shared across replicas and the notification projector's
presence gate works at N>1. Gateway + projector inject the port; isViewing is
now awaited. Presence spec updated to async (passing).

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

packages/iios-service/src/messaging/message.gateway.spec.ts

@@ -5,7 +5,7 @@ import { MessageGateway } from './message.gateway';
 import type { MessageService, MessagePrincipal } from './message.service';
 import { SessionVerifier } from '../platform/session.verifier';
 import type { OutboxBus } from '../outbox/outbox.bus';
-import type { PresenceService } from '../notifications/presence.service';
+import type { PresencePort } from '../notifications/presence.port';
 
 // Realtime delegation: the browser opens the socket with a short-lived token minted for the
 // realtime audience (iios-message). When IIOS_REALTIME_AUDIENCE is set, the gateway accepts ONLY
@@ -54,7 +54,7 @@ function gatewayReturning(principal: MessagePrincipal): MessageGateway {
     undefined as unknown as MessageService,
     session,
     undefined as unknown as OutboxBus,
-    undefined as unknown as PresenceService,
+    undefined as unknown as PresencePort,
   );
 }