Persist notifications (backend table/endpoints + triggers)

Problem

Notifications are seed-only (seedNotifications); there's no table/endpoint, mark-read mutates local state only, and nothing emits notifications on kudos/comments/conquests.

Acceptance

• Notification model + endpoints (index, mark-read, delete).
• Emit notifications on kudos/comment/conquest/defend events.
• Wire the notifications page + nav unread badge to the API.

Done (commit 57d2c46). UserNotification model + user_notifications table; kudos/comment/follow actions emit notifications for the recipient; GET /api/notifications + POST /api/notifications/read (both auth, recipient=session). Frontend: useNotifications hydrates the store, mark-all/mark-one persist via API, nav badge reflects real data. Verified end-to-end via harness.