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>

apps/api/src/app.ts

@@ -18,6 +18,7 @@ import { connectionRoutes } from './routes/connections';
 import { deviceRoutes } from './routes/devices';
 import { meRoutes } from './routes/me';
 import { reactionRoutes } from './routes/reactions';
+import { notificationRoutes } from './routes/notifications';
 import { realtimeRoutes } from './routes/realtime';
 import { safetyRoutes } from './routes/safety';
 import { trackRoutes } from './routes/tracks';
@@ -76,6 +77,7 @@ export async function buildApp({ logger = true, rateLimits = true }: { logger?:
   await app.register(adminRoutes);
   await app.register(trackRoutes);
   await app.register(reactionRoutes);
+  await app.register(notificationRoutes);
   await app.register(connectionRoutes);
   await app.register(deviceRoutes);
   await app.register(realtimeRoutes);