fix: QA findings — Dockerfile #endregion dedup, @RATIONALE, grep -qF

- Removed duplicate #endregion in backend.Dockerfile (P6 critical)
- Added @RATIONALE and @REJECTED to install_ca_to_nss and install_llm_ca_certs
- Changed grep -q to grep -qF in install_llm_ca_certs validation

docker/backend.Dockerfile

@@ -5,7 +5,6 @@
 # @RELATION DEPENDS_ON -> [EXT:path:backend/requirements.txt]
 # @PRE Docker builder context — корень проекта. backend/ и docker/ доступны.
 # @POST Образ с backend API, Playwright, entrypoint для bootstrap admin и установки сертификатов.
-# #endregion docker.backend.Dockerfile
 
 FROM python:3.11-slim