Finish Centralized NATS LLM (platform)

[suung]

Centralized NATS LLM (platform)
Branch: feature/centralized-nats-llm
Background
Main has USE_CENTRALIZED_LLM but only placeholder keys — no real chat path. Not Actwyser.
Goals

• Real NATSLLMChatAdapter in enterprise; wire analyzer when centralized mode is on.
  Acceptance criteria
• Adapter lives in report_analyst_enterprise/
• Centralized mode uses NATS, not "backend-handles-llm"
• Local get_llm() fails closed in centralized mode
• Missing enterprise package → clear ImportError
• tests/test_centralized_llm.py green