feat(iios): notification engine — presence-gated Web Push (engine phase)

Generic notification engine: reacts to message.sent and runs three gates before
delivering — policy (DM always; group only on @mention or reply-to-you), presence
(skip if the recipient is focused on that thread), mute (per-thread). Delivery via a
swappable NotificationPort (Web Push/VAPID adapter); a 'gone' (404/410) prunes the sub.

- PresenceService + gateway `focus_thread` signal + disconnect cleanup (room membership
  != viewing, since the sidebar joins every thread room).
- IiosNotificationSubscription table + `muted` on IiosThreadParticipant (migration).
- Endpoints: GET vapid-public-key, POST/DELETE subscribe, POST threads/:id/mute|unmute;
  listThreads returns the caller's `muted`.
- DM-vs-group read from the opaque `membership` attribute in the notification POLICY only
  — kernel stays generic (grep-verified).

Tests: 13 new (3 gates + reply-to-you + prune + web-push adapter states); full suite 205
green. Verified live: module boots, subscribe stored, mute/unmute round-trips.

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

packages/iios-service/src/notifications/presence.service.ts

@@ -0,0 +1,25 @@
+import { Injectable } from '@nestjs/common';
+
+/**
+ * In-memory focus tracker (single instance). Keyed on userId (the stable externalId the
+ * gateway has as principal.userId). NOTE: room membership ≠ viewing — the sidebar joins
+ * every thread room for live updates, so presence uses an explicit `focus_thread` signal.
+ * Prod (multi-replica): back this with Redis.
+ */
+@Injectable()
+export class PresenceService {
+  private readonly focus = new Map<string, { userId: string; threadId: string | null }>(); // socketId → focus
+
+  setFocus(socketId: string, userId: string, threadId: string | null): void {
+    this.focus.set(socketId, { userId, threadId });
+  }
+
+  clearSocket(socketId: string): void {
+    this.focus.delete(socketId);
+  }
+
+  isViewing(userId: string, threadId: string): boolean {
+    for (const f of this.focus.values()) if (f.userId === userId && f.threadId === threadId) return true;
+    return false;
+  }
+}