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/analytics.read-model.ts

@@ -12,12 +12,13 @@ export class IssueTimelineEntry {
   issueId!: string;
   projectId!: string;
   at!: string;
-  kind!: 'status' | 'release';
+  kind!: 'status' | 'release' | 'sprint';
   status?: string;
   category?: StatusCategory;
   /** Assignee when the status changed (throughput per person). */
   assigneeId?: string | null;
   releaseId?: string | null;
+  sprintId?: string | null;
 }
 
 export class DurationStats {