feat: due-date reminders

Every hour the API reminds whoever has a ticket, and whoever follows it,
the day before its due date, on the day and once when it is overdue, in
the app and by e-mail for those who asked. Cards on the board flag what
is due soon or late.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/notifications/reminders.spec.ts

@@ -0,0 +1,43 @@
+import {
+  IssueReadModel,
+  NotificationKind,
+} from '@task-stream/api/tracker/domain';
+import { dueStage, nextDay, remindersFor } from './reminders';
+
+const issue = (id: string, dueDate: string, watchers: string[] = []) =>
+  ({
+    id,
+    key: `WEB-${id}`,
+    title: `T${id}`,
+    projectId: 'p',
+    projectKey: 'WEB',
+    dueDate,
+    assignee: { id: 'ana', name: 'Ana' },
+    watchers: watchers.map((w) => ({ id: w, name: w })),
+  }) as IssueReadModel;
+
+describe('due reminders', () => {
+  it('tells the stage of a due date', () => {
+    expect(nextDay('2026-12-31')).toBe('2027-01-01');
+    expect(dueStage('2026-10-09', '2026-10-08')).toBe('tomorrow');
+    expect(dueStage('2026-10-08', '2026-10-08')).toBe('today');
+    expect(dueStage('2026-10-01', '2026-10-08')).toBe('overdue');
+    expect(dueStage('2026-10-20', '2026-10-08')).toBeNull();
+  });
+
+  it('reminds whoever has it and whoever follows it, once per stage', () => {
+    const reminders = remindersFor(
+      [issue('1', '2026-10-08', ['ana', 'bob']), issue('2', '2026-10-30')],
+      '2026-10-08',
+      '2026-10-08T07:00:00Z',
+    );
+    expect(reminders.map((r) => [r.userId, r.summary])).toEqual([
+      ['ana', 'WEB-1 vence hoy: T1'],
+      ['bob', 'WEB-1 vence hoy: T1'],
+    ]);
+    expect(reminders[0]).toMatchObject({
+      kind: NotificationKind.REMINDER,
+      id: 'reminder:1:2026-10-08:today:ana',
+    });
+  });
+});