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/web/tracker/domain/src/lib/models/due.spec.ts

@@ -0,0 +1,16 @@
+import { StatusCategory } from './enums';
+import { dueState } from './due';
+
+describe('due state', () => {
+  const today = '2026-10-08';
+  it('flags late, today and the next two days, never finished tickets', () => {
+    expect(dueState('2026-10-07', StatusCategory.TODO, today)).toBe('overdue');
+    expect(dueState('2026-10-08', StatusCategory.IN_PROGRESS, today)).toBe(
+      'today',
+    );
+    expect(dueState('2026-10-10', StatusCategory.QA, today)).toBe('soon');
+    expect(dueState('2026-10-11', StatusCategory.TODO, today)).toBeNull();
+    expect(dueState('2026-10-01', StatusCategory.DONE, today)).toBeNull();
+    expect(dueState(null, StatusCategory.TODO, today)).toBeNull();
+  });
+});

libs/web/tracker/domain/src/lib/models/due.ts

@@ -0,0 +1,27 @@
+import { StatusCategory } from './enums';
+
+export type DueState = 'overdue' | 'today' | 'soon' | null;
+
+/** `YYYY-MM-DD` of a local date. */
+export function localDay(date: Date): string {
+  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
+}
+
+/**
+ * How urgent a due date looks: overdue, today, or within the next two days.
+ * Finished tickets are never late.
+ */
+export function dueState(
+  dueDate: string | null,
+  category: StatusCategory,
+  today = localDay(new Date()),
+): DueState {
+  if (!dueDate) return null;
+  if (category === StatusCategory.DONE || category === StatusCategory.CANCELLED)
+    return null;
+  const due = dueDate.slice(0, 10);
+  if (due < today) return 'overdue';
+  if (due === today) return 'today';
+  const days = (Date.parse(due) - Date.parse(today)) / 86_400_000;
+  return days <= 2 ? 'soon' : null;
+}