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/issue/issue.spec.ts

@@ -279,6 +279,32 @@ describe('Issue', () => {
     });
   });
 
+  describe('backlog order', () => {
+    it('moves between two tickets, to the top, and survives a replay', () => {
+      const issue = newIssue();
+      const other = (number: number) =>
+        factory.create({
+          projectId: 'p-1',
+          projectKey: 'TS',
+          number,
+          title: `#${number}`,
+          type: IssueType.TASK,
+          reporterId: 'u',
+        });
+      const [one, two] = [other(1), other(2)];
+
+      issue.moveBetween('u-pm', one, two);
+      expect(issue.backlogRank > one.backlogRank).toBe(true);
+      expect(issue.backlogRank < two.backlogRank).toBe(true);
+      issue.moveBetween('u-pm', null, one);
+      expect(issue.backlogRank < one.backlogRank).toBe(true);
+      expect(replay(issue, Issue).rebuilt.rank).toBe(issue.rank);
+      expect(() => issue.moveBetween('u-pm', issue, null)).toThrow(
+        BusinessRuleError,
+      );
+    });
+  });
+
   describe('checklist', () => {
     it('adds, ticks, rewords and removes steps, and survives a replay', () => {
       const issue = newIssue();