feat(db): track last push time by device id with numeric timestamps

Assign stable ids to stored tokens, migrate legacy ISO lastNotifiedAt
to epoch ms, replace setLastNotifiedAt with db.update, and persist
lastNotifiedAt only after a successful FCM send. Extend Device with
optional lastNotifiedAt (ms).

src/models/device.ts

@@ -4,4 +4,6 @@ export interface Device {
   platform: "ios" | "android" | "web";
   createdAt: Date;
   updatedAt: Date;
+  /** Epoch ms; set only after a successful push send. */
+  lastNotifiedAt?: number;
 }