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/web/tracker/domain/src/lib/models/models.ts

@@ -87,6 +87,8 @@ export interface IssueSummary {
   labels: string[];
   /** The epic or ticket this one breaks down. */
   parent: IssueParent | null;
+  /** Place in the backlog and in its board column (ascending, as text). */
+  rank: string;
   commentCount: number;
   updatedAt: string;
   version: number;