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

backend/.env.example

@@ -30,8 +30,6 @@ TASKS_DATABASE_URL=postgresql+psycopg2://postgres:postgres@localhost:5432/ss_too
 # ── LLM / провайдеры ───────────────────────────────────────────────────
 OPENAI_API_KEY=
 ANTHROPIC_API_KEY=
-LLM_SSL_VERIFY=true
-LLM_CA_CERT_URLS=
 
 # ── CORS / Безопасность ────────────────────────────────────────────────
 ALLOWED_ORIGINS=http://localhost:5173,http://127.0.0.1:5173