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/read-models/issue.read-model.ts

@@ -70,6 +70,7 @@ export class IssueReadModel {
   reporter!: UserRefReadModel;
   assignee!: UserRefReadModel | null;
   release!: { id: string; name: string } | null;
+  sprint!: { id: string; name: string } | null;
   labels!: string[];
   figmaUrl!: string | null;
   storyPoints!: number | null;