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/notifications/notification.projector.ts

@@ -4,7 +4,7 @@ import { PrismaService } from '../prisma/prisma.service';
 import { OutboxBus } from '../outbox/outbox.bus';
 import { DlqService } from '../outbox/dlq.service';
 import { ProjectionCursorService } from '../projection/projection-cursor.service';
-import { PresenceService } from './presence.service';
+import { PRESENCE_PORT, type PresencePort } from './presence.port';
 import { NOTIFICATION_PORT, type NotificationPort } from './notification.port';
 
 interface MsgData {
@@ -40,7 +40,7 @@ export class NotificationProjector implements OnModuleInit {
     private readonly bus: OutboxBus,
     private readonly dlq: DlqService,
     private readonly cursor: ProjectionCursorService,
-    private readonly presence: PresenceService,
+    @Inject(PRESENCE_PORT) private readonly presence: PresencePort,
     @Inject(NOTIFICATION_PORT) private readonly port: NotificationPort,
   ) {}
 
@@ -123,7 +123,7 @@ export class NotificationProjector implements OnModuleInit {
       if (!(alwaysNotify || mentioned || repliedToMe)) continue;
 
       // gate 2 — presence
-      if (userId && this.presence.isViewing(userId, data.threadId)) continue;
+      if (userId && (await this.presence.isViewing(userId, data.threadId))) continue;
 
       // gate 3 — mute
       if (p.muted) continue;