feat(fcm): add Firebase Admin SDK and messaging export

Wire firebase-admin with ADC or FIREBASE_SERVICE_ACCOUNT_JSON,
export messaging from src/services/firebase.ts, and load it at
server startup.

package.json

@@ -9,7 +9,8 @@
     "build": "tsc"
   },
   "dependencies": {
-    "express": "^5.1.0"
+    "express": "^5.1.0",
+    "firebase-admin": "^13.9.0"
   },
   "devDependencies": {
     "@types/express": "^5.0.0",

src/index.ts

@@ -1,4 +1,5 @@
 import express from "express";
+import "./services/firebase.js";
 import { notificationsRouter } from "./routes/notifications.js";
 import { startScheduler } from "./scheduler.js";
 

src/services/firebase.ts

@@ -0,0 +1,19 @@
+import admin from "firebase-admin";
+import type { ServiceAccount } from "firebase-admin/app";
+import type { Messaging } from "firebase-admin/messaging";
+
+function resolveCredential(): admin.credential.Credential {
+  const json = process.env.FIREBASE_SERVICE_ACCOUNT_JSON;
+  if (json !== undefined && json.trim() !== "") {
+    return admin.credential.cert(JSON.parse(json) as ServiceAccount);
+  }
+  return admin.credential.applicationDefault();
+}
+
+if (!admin.apps.length) {
+  admin.initializeApp({
+    credential: resolveCredential(),
+  });
+}
+
+export const messaging: Messaging = admin.messaging();