feat(workspace): backlinks at the end of a note

GET /documents/{id}/backlinks lists the notes the user can open whose
blocks link [[Title]] (also |alias and #section, case-insensitive),
excluding the note itself and the trash. The collapsible "Verlinkt auf
diese Notiz" section shows each note with folder and the linking bullets;
a click jumps straight to the bullet.

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

frontend/src/lib/api.ts

@@ -4,6 +4,7 @@ import {
   DashboardStats,
   Document,
   TrashedDocument,
+  Backlink,
   DocumentBlock,
   DocShare,
   KanbanColumn,
@@ -653,6 +654,10 @@ export const api = {
     await request(`/api/v1/documents/${id}`, { method: 'DELETE' });
   },
 
+  async getBacklinks(id: string): Promise<Backlink[]> {
+    return request<Backlink[]>(`/api/v1/documents/${id}/backlinks`);
+  },
+
   async getTrash(): Promise<TrashedDocument[]> {
     return request<TrashedDocument[]>('/api/v1/documents/trash');
   },