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_test.go

@@ -8,13 +8,21 @@ import (
 )
 
 func TestDefaults(t *testing.T) {
-	cfg, err := Load(func(string) string { return "" })
+	cfg, err := Load(func(k string) string {
+		if k == "OLLAMA_URL" {
+			return "http://127.0.0.1:11435"
+		}
+		return ""
+	})
 	if err != nil {
 		t.Fatal(err)
 	}
 	if cfg.ListenOllama != ":11434" || cfg.ListenComfy != ":8188" {
 		t.Fatalf("listen addrs = %s %s", cfg.ListenOllama, cfg.ListenComfy)
 	}
+	if cfg.ComfyURL != "" {
+		t.Fatalf("ComfyURL default = %q, want empty (disabled)", cfg.ComfyURL)
+	}
 	if cfg.UnloadTimeout != time.Minute || cfg.JobTimeout != 15*time.Minute {
 		t.Fatalf("timeouts = %v %v", cfg.UnloadTimeout, cfg.JobTimeout)
 	}
@@ -26,6 +34,13 @@ func TestDefaults(t *testing.T) {
 	}
 }
 
+func TestLoadRequiresConsumer(t *testing.T) {
+	_, err := Load(func(string) string { return "" })
+	if err == nil || !strings.Contains(err.Error(), "OLLAMA_URL") {
+		t.Fatalf("err = %v, want missing-consumer error", err)
+	}
+}
+
 func TestParseEnvFile(t *testing.T) {
 	input := `# comment
 OLLAMA_URL=http://host:11435
@@ -91,6 +106,9 @@ func TestLoadErrors(t *testing.T) {
 			if k == tc.key {
 				return tc.value
 			}
+			if k == "OLLAMA_URL" {
+				return "http://127.0.0.1:11435"
+			}
 			return ""
 		})
 		if err == nil {