feat: PWA Phase 2 — push notifications

Add web push notification support: push_subscriptions table (migration
0007), VAPID-based push service, subscribe/unsubscribe API routes, SW
push+notificationclick handlers, and subscribe UI on account page.
Triggers: new version uploaded (all project members) and version
approved/rejected (uploader).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -6,3 +6,8 @@ S3_BUCKET=music-hub
 APP_URL=http://localhost:5173
 API_URL=http://localhost:3000
 MAGIC_LINK_SECRET=change-me-in-production
+
+# Push Notifications (VAPID)
+VAPID_PUBLIC_KEY=
+VAPID_PRIVATE_KEY=
+VAPID_EMAIL=admin@musichub.app