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/notifier.ts

@@ -60,9 +60,8 @@ export class Notifier {
         ? { id: SYSTEM_ACTOR_ID, name: SYSTEM_ACTOR_NAME }
         : refs.of(event.actorId);
 
-    const delivered: NotificationReadModel[] = [];
-    for (const draft of wanted) {
-      const notification: NotificationReadModel = {
+    await this.send(
+      wanted.map((draft) => ({
         id: `${event.aggregateId}:${versionOf(event)}:${draft.userId}`,
         userId: draft.userId,
         kind: draft.kind,
@@ -71,13 +70,22 @@ export class Notifier {
         summary: draft.summary,
         readAt: null,
         createdAt: event.occurredAt,
-      };
+      })),
+    );
+  }
+
+  /**
+   * Stores, pushes and e-mails ready-made notifications. Their id makes them
+   * idempotent: one that is already there is not sent again.
+   */
+  async send(notifications: NotificationReadModel[]): Promise<void> {
+    const delivered: NotificationReadModel[] = [];
+    for (const notification of notifications) {
       // Delivered again (at-least-once): already told, nothing to repeat.
       if (!(await this.notifications.insert(notification))) continue;
       await this.changeFeed.publishNotification(notification);
       delivered.push(notification);
     }
-
     if (this.mailer.enabled && delivered.length) {
       await this.email(delivered);
     }