Make consumers optional: a URL enables its mode, empty disables it

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.

compose.yaml.example

@@ -9,6 +9,8 @@ services:
     image: git.rambossek.at/public/gpu-turnstile:v0.1.3
     restart: unless-stopped
     environment:
+      # Each consumer is enabled by setting its URL; leave one unset to
+      # disable that side (no listener, no probe, no lock participation).
       # Services on the Docker host itself:
       OLLAMA_URL: http://host.docker.internal:11435
       COMFY_URL: http://host.docker.internal:8189