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/events/backlog.events.ts

@@ -0,0 +1,22 @@
+import { type } from '@ngrx/signals';
+import { eventGroup } from '@ngrx/signals/events';
+import { IssuePage } from '@task-stream/web/tracker/domain';
+
+export const backlogPageEvents = eventGroup({
+  source: 'Backlog Page',
+  events: {
+    opened: type<{ projectId: string }>(),
+    /** Put the ticket at `index` of the backlog as shown now. */
+    moveRequested: type<{ issueId: string; index: number }>(),
+  },
+});
+
+export const backlogApiEvents = eventGroup({
+  source: 'Backlog API',
+  events: {
+    loaded: type<IssuePage>(),
+    loadFailed: type<string>(),
+    moved: type<void>(),
+    moveFailed: type<string>(),
+  },
+});