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>
