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.ts

@@ -14,6 +14,7 @@ import { IssueCreatedEvent } from '../events/issue-created.event';
 import { IssueDeletedEvent } from '../events/issue-deleted.event';
 import { IssueLinkedEvent } from '../events/issue-linked.event';
 import { IssueParentChangedEvent } from '../events/issue-parent-changed.event';
+import { IssueRankedEvent } from '../events/issue-ranked.event';
 import { IssueReleaseChangedEvent } from '../events/issue-release-changed.event';
 import { IssueStatusChangedEvent } from '../events/issue-status-changed.event';
 import { IssueUnlinkedEvent } from '../events/issue-unlinked.event';
@@ -32,6 +33,7 @@ import {
   normalizeChecklistText,
 } from './issue-checklist';
 import { ParentCandidate, assertCanNest } from './issue-hierarchy';
+import { implicitRank, rankBetween } from './issue-rank';
 import { IssueLinkType, IssuePriority } from './issue.enums';
 
 export interface IssueLink {
@@ -108,6 +110,8 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
   /** The epic or ticket this one breaks down. */
   public parentId: string | null = null;
   public checklist: ChecklistItem[] = [];
+  /** Place in the backlog once someone moved it (see `backlogRank`). */
+  public rank: string | null = null;
   public createdAt!: string;
   public updatedAt!: string;
   public deleted = false;
@@ -316,6 +320,28 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     );
   }
 
+  /** Where it sits in the backlog: where it was put, or its creation order. */
+  get backlogRank(): string {
+    return this.rank ?? implicitRank(this.number);
+  }
+
+  /** Puts the ticket between two others of the backlog (null: an end). */
+  moveBetween(actorId: string, above: Issue | null, below: Issue | null): void {
+    for (const neighbour of [above, below]) {
+      if (neighbour?.id === this.id)
+        throw new BusinessRuleError(`${this.key} cannot move next to itself`);
+      if (neighbour && neighbour.projectId !== this.projectId)
+        throw new BusinessRuleError(
+          `${neighbour.key} belongs to another project`,
+        );
+    }
+    const rank = rankBetween(
+      above?.backlogRank ?? null,
+      below?.backlogRank ?? null,
+    );
+    this.apply(new IssueRankedEvent(actorId, this.id, this.projectId, rank));
+  }
+
   /** Adds a step to the checklist; `id` is generated by the caller. */
   addChecklistItem(actorId: string, id: string, text: string): void {
     if (this.checklist.some((i) => i.id === id)) return;
@@ -463,6 +489,11 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     this.updatedAt = event.occurredAt;
   }
 
+  onIssueRankedEvent(event: IssueRankedEvent): void {
+    // Reordering is not a change to the ticket: `updatedAt` stays.
+    this.rank = event.rank;
+  }
+
   onIssueChecklistItemAddedEvent(event: IssueChecklistItemAddedEvent): void {
     this.checklist = [...this.checklist, { ...event.item }];
     this.updatedAt = event.occurredAt;