Backbone of the entire notification stack. New notifications table (id, recipient_id, project_id, event_type enum, subject, body_md, target_url, payload jsonb, read_at, created_at). Inserts trigger Supabase Realtime broadcasts so the bell icon updates without refresh and the unread badge ticks live. Notification center popover replaces the current "all caught up" stub with a real list, mark-as-read, mark-all-read, and a /app/account/notifications page for the full archive.

Event types in v1: post.status_changed, post.commented, post.mentioned_you, changelog.published, ticket.assigned_to_you, ticket.replied, integration.error. RLS: users can only read their own rows. Out of scope: per-user preferences UI (separate card), email/push/Slack delivery (separate cards) - this card lands the spine and the in-app channel only.