feat(notifications): persist FCM tokens on POST /register

Add JSON-backed upsert store (data/fcm-tokens.json, optional
FCM_TOKEN_DATA_DIR), validate body fields, and gitignore data/.

.gitignore

@@ -2,3 +2,4 @@ node_modules/
 dist/
 .env
 *.log
+data/

src/db/fcmTokens.ts

@@ -0,0 +1,55 @@
+import { mkdir, readFile, rename, writeFile } from "node:fs/promises";
+import path from "node:path";
+
+const dataDir =
+  process.env.FCM_TOKEN_DATA_DIR ?? path.join(process.cwd(), "data");
+const dataFile = path.join(dataDir, "fcm-tokens.json");
+
+type StoredRow = {
+  fcmToken: string;
+  platform: string;
+  testMode?: boolean;
+  createdAt: string;
+  updatedAt: string;
+};
+
+async function load(): Promise<Record<string, StoredRow>> {
+  try {
+    const raw = await readFile(dataFile, "utf8");
+    return JSON.parse(raw) as Record<string, StoredRow>;
+  } catch (e: unknown) {
+    const code = (e as NodeJS.ErrnoException).code;
+    if (code === "ENOENT") return {};
+    throw e;
+  }
+}
+
+async function save(records: Record<string, StoredRow>): Promise<void> {
+  await mkdir(dataDir, { recursive: true });
+  const tmp = path.join(dataDir, `.fcm-tokens.${process.pid}.tmp`);
+  const payload = JSON.stringify(records, null, 2);
+  await writeFile(tmp, payload, "utf8");
+  await rename(tmp, dataFile);
+}
+
+export const db = {
+  async upsert(row: {
+    fcmToken: string;
+    platform: string;
+    testMode?: boolean;
+    updatedAt: Date;
+  }): Promise<void> {
+    const all = await load();
+    const key = row.fcmToken;
+    const prev = all[key];
+    const now = row.updatedAt.toISOString();
+    all[key] = {
+      fcmToken: row.fcmToken,
+      platform: row.platform,
+      testMode: row.testMode,
+      updatedAt: now,
+      createdAt: prev?.createdAt ?? now,
+    };
+    await save(all);
+  },
+};

src/routes/notifications.ts

@@ -1,7 +1,38 @@
 import { Router } from "express";
+import { db } from "../db/fcmTokens.js";
 
 export const notificationsRouter = Router();
 
 notificationsRouter.get("/", (_req, res) => {
   res.json({ ok: true, resource: "notifications" });
 });
+
+notificationsRouter.post("/register", async (req, res) => {
+  const { fcmToken, platform, testMode } = req.body as {
+    fcmToken?: unknown;
+    platform?: unknown;
+    testMode?: unknown;
+  };
+
+  if (typeof fcmToken !== "string" || fcmToken.length === 0) {
+    res.status(400).json({ error: "fcmToken is required" });
+    return;
+  }
+  if (typeof platform !== "string" || platform.length === 0) {
+    res.status(400).json({ error: "platform is required" });
+    return;
+  }
+
+  try {
+    await db.upsert({
+      fcmToken,
+      platform,
+      testMode: typeof testMode === "boolean" ? testMode : undefined,
+      updatedAt: new Date(),
+    });
+    res.sendStatus(200);
+  } catch (err) {
+    console.error("register failed", err);
+    res.sendStatus(500);
+  }
+});