Feature/fix external subscription client expiry (#6333)

* fix(sub): honor client expiry for external links

* fix(ui): show client expiry on external links

* fix(sub): address external expiry review

frontend/src/test/client-external-link-expiry.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it } from 'vitest';
+
+import { resolveExternalLinkExpiry } from '@/lib/clients/external-link';
+
+describe('resolveExternalLinkExpiry', () => {
+  it('uses the client expiry when the external link has no specific expiry', () => {
+    expect(resolveExternalLinkExpiry(0, 1_800_000_000_000)).toBe(1_800_000_000_000);
+  });
+
+  it('keeps an explicit external-link expiry', () => {
+    expect(resolveExternalLinkExpiry(1_700_000_000_000, 1_800_000_000_000)).toBe(1_700_000_000_000);
+  });
+
+  it('stays empty when neither expiry is set', () => {
+    expect(resolveExternalLinkExpiry(0, 0)).toBe(0);
+  });
+});