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/db/schema.ts

@@ -418,6 +418,15 @@ function createTables(db: Database.Database): void {
     );
     CREATE INDEX IF NOT EXISTS idx_notifications_recipient ON notifications(recipient_id, is_read, created_at DESC);
     CREATE INDEX IF NOT EXISTS idx_notifications_recipient_created ON notifications(recipient_id, created_at DESC);
+
+    CREATE TABLE IF NOT EXISTS notification_channel_preferences (
+      user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+      event_type TEXT NOT NULL,
+      channel TEXT NOT NULL,
+      enabled INTEGER NOT NULL DEFAULT 1,
+      PRIMARY KEY (user_id, event_type, channel)
+    );
+    CREATE INDEX IF NOT EXISTS idx_ncp_user ON notification_channel_preferences(user_id);
   `);
 }