fix(certs): restore HTTP CA auto-download and test edge matrix

Restore LLM_CA_CERT_URLS support for corporate PKI HTTP cert delivery.
Downloaded certificates are installed into the llm/ CA category and share
the same system trust + NSS import path as CERTS_PATH-mounted certs.

Changes:
- certs.sh: add download_llm_ca_certs with HTTP-only curl policy,
  PEM/DER handling, retries, llm/ storage, hash symlinks for llm+custom
- backend/agent entrypoints: run download before install_all_certs
- compose/env/docs: expose LLM_CA_CERT_URLS again and update ADR-0009
- integration tests: cover PEM/DER/CER, invalid certs, non-HTTP skips,
  query-string filenames, private/server skips, empty inputs, combined
  LLM_CA_CERT_URLS + CERTS_PATH channels, and NSS imports

Verified:
- 194 passed, 1 skipped integration tests
- full backend container smoke passed with testcontainers PostgreSQL
- docker bundle rebuilt for 0.5.0

docker/backend.entrypoint.sh

@@ -263,6 +263,7 @@ except Exception as exc:
 
 source "$(dirname "$0")/certs.sh"
 
+download_llm_ca_certs
 install_all_certs
 install_ca_to_nss