feat: sprints with burndown and carry-over

Plan sprints with dates and a goal, run one at a time per project, and
put tickets in them from the ticket page. The Sprints tab follows the one
under way with its progress and a burndown in tickets or points against
the ideal line. Closing a sprint sends what was left to the next one or
back to the backlog; the board can be filtered by sprint.

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

libs/web/tracker/application/src/lib/facades/issue.facade.ts

@@ -79,6 +79,13 @@ export class IssueFacade {
     );
   }
 
+  /** null: back to the backlog. */
+  setSprint(sprintId: string | null): void {
+    this.withIssue((issueId) =>
+      this.dispatch.sprintChangeRequested({ issueId, sprintId }),
+    );
+  }
+
   setRelease(releaseId: string | null): void {
     this.withIssue((issueId) =>
       this.dispatch.releaseChangeRequested({ issueId, releaseId }),