refactor(ssl): centralize SSL trust management, remove LLM_SSL_VERIFY

Centralized SSL via one contract: CERTS_PATH=/opt/certs mounted into all containers.

Backend:
  - NEW: backend/src/core/ssl.py — system_ssl_context(), httpx_verify(),
    cert_dir_inventory()
  - LLMClient._get_ssl_verify() → delegates to core.ssl
  - _llm_async_http._get_verify() → delegates to core.ssl
  - Removed LLM_SSL_VERIFY env reading from all runtime code

Docker:
  - NEW: docker/certs.sh — shared cert installer (PEM/DER/cer to .crt conversion,
    update-ca-certificates, hash symlinks, NSS import)
  - NEW: docker/agent.entrypoint.sh — agent entrypoint with cert installation
  - backend.entrypoint.sh → uses certs.sh instead of install_llm_ca_certs
  - Dockerfile.agent → adds ca-certificates, openssl, entrypoint

Compose:
  - Removed LLM_CA_CERT_URLS and LLM_SSL_VERIFY from all compose files
  - Added CERTS_PATH volume mount to agent (dev + enterprise)
  - Added certs volume mount to backend/agent in dev compose

Env examples:
  - Removed LLM_SSL_VERIFY, LLM_CA_CERT_URLS from .env.example,
    .env.enterprise-clean.example, .env.current.example, .env.master.example,
    backend/.env.example
  - Enhanced CERTS_PATH comments with accepted formats

Diagnostics:
  - diag_container.py: removed LLM_* checks, added CERTS_PATH inventory,
    uses core.ssl for context creation

Tests:
  - Updated test_llm_analysis_service, test_llm_async_http,
    test_client_headers to verify centralized ssl context (no env disable)
  - 4/4 SSL tests pass

docker-compose.yml

@@ -37,12 +37,11 @@ services:
       INITIAL_ADMIN_PASSWORD: ${INITIAL_ADMIN_PASSWORD:-}
       FEATURES__DATASET_REVIEW: ${FEATURES__DATASET_REVIEW:-true}
       FEATURES__HEALTH_MONITOR: ${FEATURES__HEALTH_MONITOR:-true}
-      LLM_CA_CERT_URLS: ${LLM_CA_CERT_URLS:-}
-      LLM_SSL_VERIFY: ${LLM_SSL_VERIFY:-true}
     ports:
       - "${BACKEND_HOST_PORT:-8001}:8000"
     volumes:
       - ./storage:/app/storage
+      - ${CERTS_PATH:-./certs}:/opt/certs:ro
 
   agent:
     build:
@@ -65,6 +64,8 @@ services:
       GRADIO_SERVER_PORT: 7860
     ports:
       - "7860"  # internal only, proxied through nginx
+    volumes:
+      - ${CERTS_PATH:-./certs}:/opt/certs:ro
 
   frontend:
     build: