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.

internal/config/config.go

@@ -51,13 +51,12 @@ type Config struct {
 }
 
 // Defaults returns the configuration used when neither the environment nor
-// a config file sets a value.
+// a config file sets a value. The upstream URLs default to empty: a
+// consumer is enabled by setting its URL, disabled by leaving it empty.
 func Defaults() Config {
 	return Config{
 		ListenOllama:   ":11434",
 		ListenComfy:    ":8188",
-		OllamaURL:      "http://127.0.0.1:11435",
-		ComfyURL:       "http://127.0.0.1:8189",
 		UnloadTimeout:  time.Minute,
 		JobTimeout:     15 * time.Minute,
 		LLMWaitTimeout: 10 * time.Minute,
@@ -219,5 +218,8 @@ func Load(getenv func(string) string) (Config, error) {
 	default:
 		return cfg, fmt.Errorf("LOG_FORMAT: must be \"text\" or \"json\"")
 	}
+	if cfg.OllamaURL == "" && cfg.ComfyURL == "" {
+		return cfg, fmt.Errorf("at least one of OLLAMA_URL or COMFY_URL must be set (each URL enables its consumer)")
+	}
 	return cfg, nil
 }