Real-time collaboration

Reports are collaborative documents. Two tables back the experience:

• report_comments — threaded, append-only, Markdown-aware.
• report_presence — heartbeat row keyed on (report_id, user_id).

The admin SPA subscribes to both via Supabase Realtime. The header shows “3 viewing” with avatar bubbles, and incoming comments stream into the panel without a refresh. Comments can be deleted by their author or by an admin — the row is soft-deleted (deleted_at) so the audit log stays intact.