feat: bulk edit tickets

Select several tickets in the backlog and change their status, assignee,
sprint, priority or labels at once. Each ticket goes through the normal
rules and may refuse on its own; the result says which did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/application/src/lib/events/backlog.events.ts

@@ -1,6 +1,10 @@
 import { type } from '@ngrx/signals';
 import { eventGroup } from '@ngrx/signals/events';
-import { IssuePage } from '@task-stream/web/tracker/domain';
+import {
+  BulkChange,
+  BulkResult,
+  IssuePage,
+} from '@task-stream/web/tracker/domain';
 
 export const backlogPageEvents = eventGroup({
   source: 'Backlog Page',
@@ -8,6 +12,7 @@ export const backlogPageEvents = eventGroup({
     opened: type<{ projectId: string }>(),
     /** Put the ticket at `index` of the backlog as shown now. */
     moveRequested: type<{ issueId: string; index: number }>(),
+    bulkRequested: type<{ issueIds: string[]; change: BulkChange }>(),
   },
 });
 
@@ -18,5 +23,6 @@ export const backlogApiEvents = eventGroup({
     loadFailed: type<string>(),
     moved: type<void>(),
     moveFailed: type<string>(),
+    bulkDone: type<BulkResult>(),
   },
 });