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/drizzle/meta/_journal.json

@@ -113,6 +113,13 @@
       "when": 1792025274882,
       "tag": "0015_lyrics",
       "breakpoints": true
+    },
+    {
+      "idx": 16,
+      "version": "7",
+      "when": 1792028149643,
+      "tag": "0016_notifications",
+      "breakpoints": true
     }
   ]
 }