feat(devices): compute effective online status with stale heartbeat ttl

Backend/utils/env.ts

@@ -32,3 +32,19 @@ export const getRequiredEnv = (name: string): string => {
 export const getBetterAuthBaseUrl = (): string => {
   return getFirstDefinedEnv('BETTER_AUTH_BASE_URL', 'BETTER_AUTH_URL') ?? `http://localhost:${process.env.PORT ?? '3000'}`;
 };
+
+const DEFAULT_DEVICE_ONLINE_STALE_SECONDS = 30;
+
+export const getDeviceOnlineStaleSeconds = (): number => {
+  const value = getFirstDefinedEnv('DEVICE_ONLINE_STALE_SECONDS');
+  if (!value) {
+    return DEFAULT_DEVICE_ONLINE_STALE_SECONDS;
+  }
+
+  const parsed = Number(value);
+  if (!Number.isInteger(parsed) || parsed <= 0) {
+    return DEFAULT_DEVICE_ONLINE_STALE_SECONDS;
+  }
+
+  return parsed;
+};