Keep an offline endpoint's models in /v1/models, flagged endpoint_offline #35

User story

As an operator, I want relayLLM to keep an offline endpoint's models in its model list, flagged offline, so that relay can answer "endpoint unavailable" instead of "model not found" during a short outage.

Acceptance criteria

• GET /v1/models keeps each last-known model of an offline endpoint, with endpoint_offline set to the endpoint name.
• GET /v1/models keeps each modelMap key whose target is such a model, with the same endpoint_offline value.
• Managed and virtual rows never carry endpoint_offline.
• The offline mark clears within the existing 15 s probe window.

What I'd notice

A clearer error during a brief endpoint outage, once relay reads the flag.

Context

barelyworkingcode/relay#497 adds the relay half: relay reads endpoint_offline and answers 503 naming the endpoint. Today relayLLM drops an offline endpoint's rows, so relay cannot tell offline from absent. The wire shape is in the ## Contract of barelyworkingcode/relay#497. Found while working barelyworkingcode/relay#497.

Tasks

• Keep last-known rows and their modelMap keys with endpoint_offline.
• Test both kinds of row and the clear.
• Real-app check against relay main with relay#497 merged.