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/domain/src/lib/models/enums.ts

@@ -39,6 +39,13 @@ export enum ReleaseStatus {
   ARCHIVED = 'ARCHIVED',
 }
 
+export enum SprintStatus {
+  PLANNED = 'PLANNED',
+  /** Under way: one per project at a time. */
+  ACTIVE = 'ACTIVE',
+  CLOSED = 'CLOSED',
+}
+
 /** Why someone is told. */
 export enum NotificationKind {
   ASSIGNED = 'ASSIGNED',