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/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -64,6 +64,12 @@ export interface GitIntegrationDoc {
   secret?: string;
   createdAt?: string;
 }
+export interface CalendarFeedDoc {
+  /** The user's id. */
+  id: string;
+  token: string;
+  createdAt: string;
+}
 export interface NotificationSettingsDoc {
   /** The user's id. */
   id: string;
@@ -85,6 +91,7 @@ export const ReadModels = {
   OutgoingWebhook: 'OutgoingWebhook',
   Automation: 'Automation',
   AutomationRun: 'AutomationRun',
+  CalendarFeed: 'CalendarFeed',
 } as const;
 
 /*
@@ -329,6 +336,17 @@ class AutomationRunDocument {
 export const AutomationRunSchema = SchemaFactory.createForClass(
   AutomationRunDocument,
 );
+/** One per person: the secret of their calendar URL. */
+@Schema({ ...LOOSE, collection: 'calendar_feeds' })
+class CalendarFeedDocument {
+  /** The user's id. */
+  @Prop({ type: String, required: true, unique: true }) id!: string;
+  @Prop({ type: String, required: true, unique: true }) token!: string;
+  @Prop({ type: String }) createdAt!: string;
+}
+export const CalendarFeedSchema =
+  SchemaFactory.createForClass(CalendarFeedDocument);
+
 // Redeliveries happen within minutes; a month of memory is plenty.
 AutomationRunSchema.index({ at: 1 }, { expireAfterSeconds: 30 * 86_400 });
 
@@ -347,6 +365,7 @@ export const READ_MODEL_SCHEMAS = [
   { name: ReadModels.Notification, schema: NotificationSchema },
   { name: ReadModels.NotificationSettings, schema: NotificationSettingsSchema },
   { name: ReadModels.SavedFilter, schema: SavedFilterSchema },
+  { name: ReadModels.CalendarFeed, schema: CalendarFeedSchema },
 ];
 
 /** A document as the model it holds: Mongo's own `_id` left out. */