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/ports/gateways.ts

@@ -2,21 +2,16 @@ import { Observable } from 'rxjs';
 import {
   Activity,
   Attachment,
+  Automation,
+  AutomationInput,
   BoardColumn,
+  CalendarFeed,
+  CommandResult,
   Comment,
   Epic,
   GitIntegration,
   GitRepository,
-  Automation,
-  AutomationInput,
   ImportReport,
-  Webhook,
-  WebhookDelivery,
-  SavedFilter,
-  Sprint,
-  SprintBurndown,
-  SavedFilterCriteria,
-  CommandResult,
   Issue,
   IssuePage,
   IssueSummary,
@@ -27,7 +22,13 @@ import {
   ProjectStats,
   Release,
   ReleaseBurnup,
+  SavedFilter,
+  SavedFilterCriteria,
+  Sprint,
+  SprintBurndown,
   UserDirectoryEntry,
+  Webhook,
+  WebhookDelivery,
 } from '../models/models';
 import { ProjectSettings } from '../models/workflow';
 import {
@@ -332,6 +333,14 @@ export abstract class UsersGateway {
 }
 
 /** The signed-in person's notifications. */
+export abstract class CalendarGateway {
+  /** Mine, if I made one. */
+  abstract mine(): Observable<CalendarFeed | null>;
+  /** A new secret URL; the previous one stops working. */
+  abstract create(): Observable<CalendarFeed>;
+  abstract revoke(): Observable<void>;
+}
+
 export abstract class NotificationsGateway {
   abstract mine(
     unreadOnly: boolean,