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/web/tracker/application/src/lib/stores/board.reducers.ts

@@ -29,7 +29,18 @@ export function matchesFilter(
   return true;
 }
 
-/** Removes the issue wherever it is and puts `next` (if any) on top of its status column. */
+/** `issues` (in backlog order) with `issue` at its place by rank. */
+export function insertByRank(
+  issues: IssueSummary[],
+  issue: IssueSummary,
+): IssueSummary[] {
+  const at = issues.findIndex((i) => i.rank > issue.rank);
+  return at === -1
+    ? [...issues, issue]
+    : [...issues.slice(0, at), issue, ...issues.slice(at)];
+}
+
+/** Removes the issue wherever it is and puts `next` (if any) at its place by rank in its status column. */
 export function placeIssue(
   columns: BoardColumn[],
   issueId: string,
@@ -42,7 +53,7 @@ export function placeIssue(
       : column.issues;
     let total = column.total - (had ? 1 : 0);
     if (next && next.status === column.status) {
-      issues = [next, ...issues];
+      issues = insertByRank(issues, next);
       total += 1;
     }
     return had || issues !== column.issues