Notifications and sending songs to friends

Reactions to your songs, new followers, and songs sent to you are now
notifications: a bell on the feed (app and website) with an unread
count that updates live, and a list that marks them read. "Send ✉" on
a song sends it to people who follow you, with an optional note.
Settings picks which kinds to be notified about.

Push delivery uses no third-party accounts:
- Android: UnifiedPush through a distributor app (ntfy), pointed at
  our own ntfy server (docker-compose.ntfy.yml). Works the same in
  the F-Droid build; no Firebase.
- Browsers: standard Web Push from the existing service worker.
Both use Web Push encryption with a VAPID key pair the server makes
and keeps in the database. Push addresses are checked like any other
user-supplied URL. Setup is in docs/PUSH.md; the privacy page covers
the new data.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/shared/src/dto.ts

@@ -1,4 +1,5 @@
 import type { Entitlements, Plan } from './plans';
+import type { NotificationPrefs } from './notifications';
 import type { NowPlayingProvider, StreamingService } from './services';
 
 export interface TrackDTO {
@@ -28,6 +29,7 @@ export interface MeDTO extends UserSummaryDTO {
   preferredService: StreamingService | null;
   /** The services they use; null until chosen, meaning all of them. */
   services: StreamingService[] | null;
+  notifyPrefs: NotificationPrefs;
   role: 'user' | 'admin';
   /** When onboarding was finished; null means show it. */
   onboardedAt: string | null;
@@ -161,4 +163,7 @@ export interface TrackSearchResultDTO {
 }
 
 /** Messages pushed over the realtime websocket. */
-export type RealtimeMessage = { type: 'now_playing'; data: NowPlayingDTO };
+export type RealtimeMessage =
+  | { type: 'now_playing'; data: NowPlayingDTO }
+  /** Something new in the viewer's notifications. */
+  | { type: 'notification'; data: { unread: number } };