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/application/src/lib/commands/rank-issue.command.ts

@@ -0,0 +1,17 @@
+import { Command } from '@nestjs/cqrs';
+import { CommandResult } from './command-result';
+
+/**
+ * Moves a ticket in the backlog, between the one that ends up above it and the
+ * one that ends up below (null: the top or the bottom).
+ */
+export class RankIssueCommand extends Command<CommandResult> {
+  constructor(
+    public readonly actorId: string,
+    public readonly issueId: string,
+    public readonly aboveIssueId: string | null,
+    public readonly belowIssueId: string | null,
+  ) {
+    super();
+  }
+}