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.ts

@@ -1,5 +1,5 @@
 import { randomUUID } from 'node:crypto';
-import { Logger } from '@nestjs/common';
+import { Inject, Logger } from '@nestjs/common';
 import {
   ConnectedSocket,
   MessageBody,
@@ -16,7 +16,7 @@ import { logJson } from '../observability/logger';
 import { MessageService, type MessagePrincipal } from './message.service';
 import { SessionVerifier } from '../platform/session.verifier';
 import { OutboxBus } from '../outbox/outbox.bus';
-import { PresenceService } from '../notifications/presence.service';
+import { PRESENCE_PORT, type PresencePort } from '../notifications/presence.port';
 
 interface SocketState {
   principal: MessagePrincipal;
@@ -39,7 +39,7 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection, OnGat
     private readonly messages: MessageService,
     private readonly session: SessionVerifier,
     private readonly bus: OutboxBus,
-    private readonly presence: PresenceService,
+    @Inject(PRESENCE_PORT) private readonly presence: PresencePort,
   ) {}
 
   afterInit(): void {
@@ -77,7 +77,8 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection, OnGat
   }
 
   handleDisconnect(client: Socket): void {
-    this.presence.clearSocket(client.id);
+    // Fire-and-forget: presence is best-effort and must not block the disconnect path.
+    void this.presence.clearSocket(client.id).catch((err) => this.logger.warn(`presence clear failed: ${(err as Error).message}`));
   }
 
   /** The app reports which thread is in the foreground (or null when blurred) — presence. */
@@ -85,7 +86,9 @@ export class MessageGateway implements OnGatewayInit, OnGatewayConnection, OnGat
   focusThread(@ConnectedSocket() client: Socket, @MessageBody() body: { threadId: string | null }): void {
     const state = client.data as SocketState | undefined;
     if (!state?.principal) return;
-    this.presence.setFocus(client.id, state.principal.userId, body?.threadId ?? null);
+    void this.presence
+      .setFocus(client.id, state.principal.userId, body?.threadId ?? null)
+      .catch((err) => this.logger.warn(`presence set failed: ${(err as Error).message}`));
   }
 
   @SubscribeMessage('open_thread')