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/infrastructure/src/lib/graphql/issues.graphql

@@ -176,3 +176,18 @@ mutation RemoveChecklistItem($issueId: ID!, $itemId: ID!) {
     ...CommandResultFields
   }
 }
+
+query Backlog($filter: IssueFilterInput!) {
+  issues(filter: $filter, limit: 200) {
+    items {
+      ...IssueSummaryFields
+    }
+    total
+  }
+}
+
+mutation RankIssue($input: RankIssueInput!) {
+  rankIssue(input: $input) {
+    ...CommandResultFields
+  }
+}