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

Backend/tests/env.test.ts

@@ -1,6 +1,6 @@
 import { afterEach, describe, expect, test } from 'bun:test';
 
-import { getBetterAuthBaseUrl, getFirstDefinedEnv } from '../utils/env';
+import { getBetterAuthBaseUrl, getDeviceOnlineStaleSeconds, getFirstDefinedEnv } from '../utils/env';
 
 const ORIGINAL_ENV = { ...process.env };
 
@@ -22,4 +22,25 @@ describe('env helpers', () => {
 
     expect(getBetterAuthBaseUrl()).toBe('http://base-url:4000');
   });
+
+  test('getDeviceOnlineStaleSeconds defaults to 30', () => {
+    delete process.env.DEVICE_ONLINE_STALE_SECONDS;
+    expect(getDeviceOnlineStaleSeconds()).toBe(30);
+  });
+
+  test('getDeviceOnlineStaleSeconds parses valid positive integer values', () => {
+    process.env.DEVICE_ONLINE_STALE_SECONDS = '45';
+    expect(getDeviceOnlineStaleSeconds()).toBe(45);
+  });
+
+  test('getDeviceOnlineStaleSeconds falls back to default on invalid values', () => {
+    process.env.DEVICE_ONLINE_STALE_SECONDS = '0';
+    expect(getDeviceOnlineStaleSeconds()).toBe(30);
+
+    process.env.DEVICE_ONLINE_STALE_SECONDS = '-2';
+    expect(getDeviceOnlineStaleSeconds()).toBe(30);
+
+    process.env.DEVICE_ONLINE_STALE_SECONDS = 'abc';
+    expect(getDeviceOnlineStaleSeconds()).toBe(30);
+  });
 });