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/domain/src/lib/events/issue-sprint-changed.event.ts

@@ -0,0 +1,16 @@
+import { AutowiredEvent } from '@task-stream/api/shared/domain';
+import { TrackerEvent } from './tracker.event';
+
+/** A ticket was moved in or out of a sprint. */
+@AutowiredEvent
+export class IssueSprintChangedEvent extends TrackerEvent {
+  constructor(
+    actorId: string,
+    issueId: string,
+    projectId: string,
+    readonly sprintId: string | null,
+    readonly previousSprintId: string | null,
+  ) {
+    super(actorId, issueId, projectId);
+  }
+}