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/services/vacayService.ts

@@ -342,8 +342,8 @@ export function sendInvite(planId: number, inviterId: number, inviterUsername: s
   } catch { /* websocket not available */ }
 
   // Notify invited user
-  import('../services/notifications').then(({ notify }) => {
-    notify({ userId: targetUserId, event: 'vacay_invite', params: { actor: inviterEmail } }).catch(() => {});
+  import('../services/notificationService').then(({ send }) => {
+    send({ event: 'vacay_invite', actorId: inviterId, scope: 'user', targetId: targetUserId, params: { actor: inviterEmail, planId: String(planId) } }).catch(() => {});
   });
 
   return {};