feat(notifications): add unified multi-channel notification system

Introduces a fully featured notification system with three delivery
channels (in-app, email, webhook), normalized per-user/per-event/
per-channel preferences, admin-scoped notifications, scheduled trip
reminders and version update alerts.

- New notificationService.send() as the single orchestration entry point
- In-app notifications with simple/boolean/navigate types and WebSocket push
- Per-user preference matrix with normalized notification_channel_preferences table
- Admin notification preferences stored globally in app_settings
- Migration 69 normalizes legacy notification_preferences table
- Scheduler hooks for daily trip reminders and version checks
- DevNotificationsPanel for testing in dev mode
- All new tests passing, covering dispatch, preferences, migration, boolean
  responses, resilience, and full API integration (NSVC, NPREF, INOTIF,
  MIGR, VNOTIF, NROUTE series)
 - Previous tests passing

server/src/index.ts

@@ -46,6 +46,7 @@ const server = app.listen(PORT, () => {
   }
   scheduler.start();
   scheduler.startTripReminders();
+  scheduler.startVersionCheck();
   scheduler.startDemoReset();
   const { startTokenCleanup } = require('./services/ephemeralTokens');
   startTokenCleanup();