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/events/index.ts

@@ -30,3 +30,4 @@ export * from './issue-parent-changed.event';
 export * from './issue-checklist-item-added.event';
 export * from './issue-checklist-item-updated.event';
 export * from './issue-checklist-item-removed.event';
+export * from './issue-ranked.event';

libs/api/tracker/domain/src/lib/events/issue-ranked.event.ts

@@ -0,0 +1,15 @@
+import { AutowiredEvent } from '@task-stream/api/shared/domain';
+import { TrackerEvent } from './tracker.event';
+
+/** A ticket was moved to another place in the backlog. */
+@AutowiredEvent
+export class IssueRankedEvent extends TrackerEvent {
+  constructor(
+    actorId: string,
+    issueId: string,
+    projectId: string,
+    readonly rank: string,
+  ) {
+    super(actorId, issueId, projectId);
+  }
+}