feat: an ordered backlog

A Backlog tab lists the tickets still to start in the order the team will
take them; drag them, or use the arrows, to reorder. Moving a ticket only
rewrites that ticket's rank, read from the event store, and board columns
follow the same order.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/domain/src/lib/read-models/issue.read-model.ts

@@ -79,6 +79,8 @@ export class IssueReadModel {
   /** People following the ticket. */
   watchers!: UserRefReadModel[];
   parent!: IssueParentReadModel | null;
+  /** Place in the backlog (and in each board column): ascending. */
+  rank!: string;
   checklist!: ChecklistItemReadModel[];
   commentCount!: number;
   createdAt!: string;