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/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -7,6 +7,7 @@ import {
   IssueTimelineEntry,
   ProjectReadModel,
   ReleaseReadModel,
+  implicitRank,
 } from '@task-stream/api/tracker/domain';
 import { Schema } from 'mongoose';
 
@@ -102,6 +103,7 @@ IssueSchema.index({ projectId: 1, statusCategory: 1 });
 IssueSchema.index({ 'assignee.id': 1 });
 IssueSchema.index({ 'watchers.id': 1 });
 IssueSchema.index({ 'parent.id': 1 });
+IssueSchema.index({ projectId: 1, rank: 1 });
 IssueSchema.index({ 'release.id': 1 });
 IssueSchema.index({ title: 'text', description: 'text', key: 'text' });
 
@@ -186,6 +188,7 @@ export function issueFromDoc(doc: IssueDoc): IssueReadModel {
     attachments: rest.attachments ?? [],
     watchers: rest.watchers ?? [],
     parent: rest.parent?.id ? rest.parent : null,
+    rank: rest.rank ?? implicitRank(rest.number),
     checklist: rest.checklist ?? [],
   };
 }