feat(register): key devices by deviceId and replace FCM tokens in place

Require deviceId on POST /notifications/register, upsert by deviceId
while preserving lastNotifiedAt and internal id, prune duplicate token
rows, migrate legacy fcmToken-keyed JSON, and add register logs.
Extend StoredRow and Device with deviceId; resolve pushes by scanning
fcmToken.

src/models/device.ts

@@ -1,6 +1,9 @@
 export interface Device {
+  /** Internal row id used for persistence updates. */
   id: string;
-  pushToken: string;
+  /** Client-provided stable physical device identity. */
+  deviceId: string;
+  fcmToken: string;
   platform: "ios" | "android" | "web";
   createdAt: Date;
   updatedAt: Date;