Email + browser push notifications

[sjcotto]

Notifications are in-app only (Inbox via lib/notifications.ts + components/sidebar/inbox.tsx): mention / assigned / comment.

Scope

• Email delivery (pluggable provider) for notifications, with per-user preferences and digests.
• Web Push (service worker) for real-time browser notifications.
• Respect a quiet-hours / per-type preference model.

Acceptance

• Getting @mentioned sends an email and/or a browser push per the user's settings.