site.yml's Frontend play references a nonexistent role frontend_app #16342

What

`autobot-slm-backend/ansible/site.yml`'s "Deploy Frontend Services" play (`hosts: frontend`) lists:

```yaml
roles:

• nodejs
• nginx
• frontend_app
  ```

`roles/frontend_app/` does not exist under `autobot-slm-backend/ansible/roles/`. The real role is `roles/frontend`. Any `ansible-playbook site.yml` run against a `frontend`-group host fails at that play (unknown role).

How found

Found while verifying #16310's fix reachability: the backend play in the same file already got a similar fix once (see the comment above the Backend play, "#10048: use the maintained roles... the old list referenced nonexistent roles"). The Frontend play was apparently missed at that time.

`autobot-slm-backend/ansible/playbooks/provision-fleet-roles.yml`'s "Provision Phase 4b: Frontend" play correctly includes `roles/frontend` and is confirmed to be frontend's real, working provisioning path -- so this is not a live-fleet blocker today, but `site.yml` itself is broken for this play and should either be fixed (rename to `frontend`) or retired if `provision-fleet-roles.yml` has fully superseded it.

Evidence

• `autobot-slm-backend/ansible/site.yml`: Frontend play roles list
• `autobot-slm-backend/ansible/roles/`: no `frontend_app` directory, only `frontend`
• Guard added in deploy: the builtin updater never removes files deleted from source, so deleted modules pile up on every host #16310's PR: `repo_tests/sync_deletions_ansible_wiring_16310_test.py::test_site_yml_frontend_role_reference_is_broken` documents this as a stated (not silent) gap.

Suggested fix

Change `frontend_app` to `frontend` in `site.yml`, or confirm `site.yml` is deprecated in favor of `provision-fleet-roles.yml` and remove/deprecate the stale play.