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

@@ -429,6 +429,13 @@ export interface Notification {
   createdAt: string;
 }
 
+/** My calendar subscription: a secret URL calendar apps read (iCalendar). */
+export interface CalendarFeed {
+  /** Absolute: what goes into Google Calendar, Outlook or Apple Calendar. */
+  url: string;
+  createdAt: string;
+}
+
 export interface NotificationSettings {
   /** Copies by e-mail. */
   email: boolean;