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/models/device.ts

@@ -0,0 +1,7 @@
+export interface Device {
+  id: string;
+  pushToken: string;
+  platform: "ios" | "android" | "web";
+  createdAt: Date;
+  updatedAt: Date;
+}