feat(backend): scaffold Express API with health and module layout

Add src/routes/notifications, services/pushService, models/device,
scheduler stubs, and entrypoint with GET /health and startup log.

src/scheduler.ts

@@ -0,0 +1,16 @@
+let intervalId: ReturnType<typeof setInterval> | undefined;
+
+export function startScheduler(): void {
+  if (intervalId !== undefined) return;
+  // TODO: replace with job queue or cron for wake-up checks
+  intervalId = setInterval(() => {
+    // placeholder tick
+  }, 60_000);
+}
+
+export function stopScheduler(): void {
+  if (intervalId !== undefined) {
+    clearInterval(intervalId);
+    intervalId = undefined;
+  }
+}