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/api/tracker/application/src/lib/changes.service.ts

@@ -8,6 +8,7 @@ import {
   IssueChange,
   ProjectChange,
   ReleaseChange,
+  SprintChange,
 } from './ports/change-feed';
 import { CanViewProjectQuery } from './queries/can-view-project.query';
 import { GetIssueQuery } from './queries/get-issue.query';
@@ -52,6 +53,14 @@ export class ChangesService {
     return this.changeFeed.releases(projectId);
   }
 
+  async watchSprints(
+    viewerId: string,
+    projectId: string,
+  ): Promise<AsyncIterableIterator<SprintChange>> {
+    await this.assertCanView(viewerId, projectId);
+    return this.changeFeed.sprints(projectId);
+  }
+
   async watchProject(
     viewerId: string,
     projectId: string,