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/package.json

@@ -36,11 +36,13 @@
     "postgres": "^3.4.9",
     "sharp": "^0.35.5",
     "undici": "^6.29.0",
+    "web-push": "^3.6.7",
     "zod": "^4.6.5"
   },
   "devDependencies": {
     "@types/node": "^26.6.4",
     "@types/nodemailer": "^8.0.2",
+    "@types/web-push": "^3.6.4",
     "@types/ws": "^8.18.2",
     "concurrently": "^10.0.5",
     "drizzle-kit": "^0.31.11",