feat(tokens): allow permanent deletion of revoked tokens

Add POST /admin/tokens/{id}/delete endpoint that permanently removes
a token from the database (only if already revoked). Add delete button
in backoffice UI for revoked tokens.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/lib/api.ts

@@ -254,6 +254,10 @@ export async function revokeToken(id: string) {
   return apiFetch<void>(`/admin/tokens/${id}`, { method: "DELETE" });
 }
 
+export async function deleteToken(id: string) {
+  return apiFetch<void>(`/admin/tokens/${id}/delete`, { method: "POST" });
+}
+
 export async function fetchBooks(
   libraryId?: string,
   series?: string,