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/notification.dto.ts

@@ -0,0 +1,12 @@
+import { IsIn, IsObject, IsOptional, IsString } from 'class-validator';
+
+export class SubscribeDto {
+  @IsOptional() @IsIn(['webpush']) kind?: string;
+  @IsString() endpoint!: string;
+  @IsObject() keys!: { p256dh: string; auth: string };
+  @IsOptional() @IsString() userAgent?: string;
+}
+
+export class UnsubscribeDto {
+  @IsString() endpoint!: string;
+}