feat(deadline-monitor): scheduled contract deadline monitor

[modecodes]

Summary

Adds a scheduled service that monitors milestone deadlines so overdue work and approaching due dates are surfaced automatically, instead of relying on someone checking manually. This is a net-new feature — no existing deadline-tracking service was in place.

What was built

• lib/deadline-monitor/service.ts — DeadlineMonitorService.runCheck() runs two independent, idempotent sweeps:

  1. Upcoming deadline reminders — milestones due within the next 24 hours that haven't been reminded yet get a one-time notification sent to both the client and the freelancer.
  2. Overdue flagging — milestones whose due date has passed get flagged is_overdue = true and both parties are notified.

  Both sweeps skip milestones already in a terminal status (approved, paid, rejected), and both are guarded by bookkeeping columns (reminder_sent_at, is_overdue) so re-running the sweep on a tight interval never sends duplicate notifications.

• lib/notifications.ts — extended NotificationType with milestone_deadline_approaching and milestone_overdue, so these events reuse the existing notifications table/API/pagination instead of introducing a parallel event log.

• components/dashboard/notification-item.tsx — added icon/label/description handling for the two new notification types so they render properly in the dashboard notification panel.

• scripts/deadline-monitor.ts — standalone worker process (npm run deadline-monitor), following the same long-running-process pattern as the existing scripts/sync-worker.ts. Runs an initial check on startup, then repeats on an interval controlled by DEADLINE_CHECK_INTERVAL_MS (default 5 minutes).

• lib/db/migrations/007_deadline_monitor.sql — adds is_overdue, overdue_at, reminder_sent_at to milestones, plus a partial index for the overdue sweep query.

• docs/deadline-monitor.md — architecture, configuration, and testing docs.

Acceptance criteria

• ✅ Accurate detection — overdue milestones are identified via a due_date < NOW() sweep, excluding already-terminal milestones.
• ✅ Timely notifications — reminders fire 24h before the deadline; overdue notices fire as soon as the sweep runs past the due date.
• ✅ Automated status updates — milestones are automatically flagged is_overdue with an overdue_at timestamp.
• ✅ Reminder events generated — both reminder and overdue events are logged via the notifications system and surfaced in the existing dashboard notification panel.

Testing

Added __tests__/deadline-monitor/service.test.ts and types.test.ts, covering: no-op behavior when nothing is due, reminder notification + bookkeeping, overdue flagging + notification, and both sweeps running independently within a single runCheck() call. Full suite passes; no new lint or type errors introduced.

Closes #153