fix(backend): DELETE /webhooks/:id returns 204 No Content

Matches REST convention and the frontend api-client's existing 204
fast-path, avoiding a JSON-parse crash on an empty response body.

ApiCore-Backend/test/webhooks.e2e-spec.ts

@@ -170,6 +170,6 @@ describe('Webhooks (e2e)', () => {
     return request(app.getHttpServer())
       .delete(`/api/v1/webhooks/${subscriptionId}`)
       .set('Authorization', `Bearer ${adminAccessToken}`)
-      .expect(200);
+      .expect(204);
   });
 });