OLLAMA_URL and COMFY_URL no longer have defaults; each consumer
(listener, client, startup probe, lock participation) is enabled by
setting its URL and disabled by leaving it empty. At least one must be
set. Ollama-only mode is a pure pass-through; ComfyUI-only mode skips
the unload and warm-reload steps. /metrics is now served on both
listeners. This is the extension pattern for future consumers such as
local game detection.