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/domain/src/lib/ports/gateways.ts

@@ -101,6 +101,14 @@ export abstract class IssuesGateway {
     parentId: string | null,
   ): Observable<CommandResult>;
   abstract epics(projectId: string): Observable<Epic[]>;
+  /** Tickets still to start (TODO category), in backlog order. */
+  abstract backlog(projectId: string): Observable<IssuePage>;
+  /** Moves a ticket between two others of the backlog (null: an end). */
+  abstract rank(
+    issueId: string,
+    aboveIssueId: string | null,
+    belowIssueId: string | null,
+  ): Observable<CommandResult>;
   abstract addChecklistItem(
     issueId: string,
     text: string,