feat: a sprint calendar

A Calendario tab lays the project's sprints out month by month as bands,
with the releases and the tickets due on each day. Owners and admins plan
a sprint by clicking the day it starts, and the whole plan downloads as an
iCalendar file. Tickets can be searched by due-date range.

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

libs/web/tracker/domain/src/lib/ports/gateways.ts

@@ -115,6 +115,12 @@ export abstract class IssuesGateway {
     parentId: string | null,
   ): Observable<CommandResult>;
   abstract epics(projectId: string): Observable<Epic[]>;
+  /** Tickets due between two days (`YYYY-MM-DD`, both included). */
+  abstract due(
+    projectId: string,
+    from: string,
+    to: string,
+  ): Observable<IssueSummary[]>;
   /** To the trash (whoever opened it, or a manager). */
   abstract trash(issueId: string): Observable<CommandResult>;
   abstract restore(issueId: string): Observable<CommandResult>;