✨ Notifications: in-app alerts and email digest

[fusion94]

Summary

The community has no way to know when interesting things happen. Add a lightweight notification system covering the two highest-value events — invite usage and new community events — surfaced as an in-app badge plus an opt-in email digest.

Depends on #20 (Resend) for the email side.

Expected Behavior

• New table notifications(id, recipient, type, payload, created_at, read_at).
• Notifications generated server-side when:
  • Someone registers using your invite token (notify the inviter).
  • A community event is created (notify all members).
• AppBar gets a bell icon with an unread count badge.
• Clicking it opens a dropdown listing recent notifications with click-through links (e.g. to the event).
• Profile page exposes a "Daily digest email" toggle — when on, Resend job sends a once-a-day summary of unread notifications.

Proposed Approach

1. Add notifications table and helper createNotification(recipient, type, payload).
2. Wire createNotification into the register handler (server/index.js:209) and event-create handler (server/index.js:1076).
3. Add GET /api/notifications (auth) and POST /api/notifications/:id/read.
4. Frontend: bell icon in Layout.jsx, dropdown component, polling every 60s while tab is focused.
5. Daily digest: cron-style scheduled task on the server (or external cron hitting an admin endpoint), iterate users with digest_enabled = 1 and unread notifications, send via Resend.

Acceptance Criteria

• Inviter gets notified when their invite is used.
• All members get notified on community event creation.
• Bell icon shows unread count, dropdown lists notifications, marking-read works.
• Profile has a "daily digest" toggle stored per user.
• Digest job runs once per day and skips users with no unread notifications.