feat: a calendar to subscribe to, and the backlog's corners

- Each person can subscribe their dates in Google Calendar, Outlook or
  Apple Calendar: GET /api/calendar/{token}.ics (no session) brings
  their unfinished tickets with a due date (assigned or followed), the
  planned and running sprints and the planned releases of their
  projects, as all-day events linking to the web. The token is made,
  renewed or dropped under /me/calendar (and in GraphQL); in the web,
  from «Calendario» in «Mi trabajo».
- The backlog panel keeps the selected rows' background inside its
  rounded corners.

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

libs/api/tracker/application/src/lib/calendar.service.ts

@@ -0,0 +1,36 @@
+import { Injectable } from '@nestjs/common';
+import { CommandBus, QueryBus } from '@nestjs/cqrs';
+import {
+  CreateCalendarFeedCommand,
+  GetCalendarIcsQuery,
+  GetMyCalendarFeedQuery,
+  RevokeCalendarFeedCommand,
+} from './calendar/calendar';
+import { CalendarFeed } from './ports/calendar-feeds.repository';
+
+/** A person's calendar subscription (iCalendar), behind a secret URL. */
+@Injectable()
+export class CalendarService {
+  constructor(
+    private readonly commandBus: CommandBus,
+    private readonly queryBus: QueryBus,
+  ) {}
+
+  mine(userId: string): Promise<CalendarFeed | null> {
+    return this.queryBus.execute(new GetMyCalendarFeedQuery(userId));
+  }
+
+  /** A new secret URL; the previous one, if any, stops working. */
+  create(userId: string): Promise<CalendarFeed> {
+    return this.commandBus.execute(new CreateCalendarFeedCommand(userId));
+  }
+
+  revoke(userId: string): Promise<void> {
+    return this.commandBus.execute(new RevokeCalendarFeedCommand(userId));
+  }
+
+  /** The calendar behind `token`, or null if no feed has it. */
+  ics(token: string, webUrl: string): Promise<string | null> {
+    return this.queryBus.execute(new GetCalendarIcsQuery(token, webUrl));
+  }
+}