fix: fallback to /app config when /data is unavailable

internal/config/paths.go

@@ -31,13 +31,20 @@ func ResolvePath(envKey, defaultRel string) string {
 
 func ConfigPath() string {
 	if strings.TrimSpace(os.Getenv("DS2API_CONFIG_PATH")) == "" && BaseDir() == "/app" {
-		// Official container images commonly run from /app where filesystem may be read-only.
-		// Prefer /data default so deployments can persist config/token state by mounting a volume.
-		return "/data/config.json"
+		return containerDefaultConfigPath()
 	}
 	return ResolvePath("DS2API_CONFIG_PATH", "config.json")
 }
 
+func containerDefaultConfigPath() string {
+	// Container images run as non-root by default. Only use /data when mounted/provisioned.
+	// Otherwise keep /app/config.json so admin-side save does not fail on MkdirAll("/data").
+	if st, err := os.Stat("/data"); err == nil && st.IsDir() {
+		return "/data/config.json"
+	}
+	return "/app/config.json"
+}
+
 func legacyContainerConfigPath() string {
 	return "/app/config.json"
 }

internal/config/paths_test.go

@@ -0,0 +1,28 @@
+package config
+
+import (
+	"os"
+	"testing"
+)
+
+func TestContainerDefaultConfigPath(t *testing.T) {
+	t.Run("fallback to /app when /data is missing", func(t *testing.T) {
+		// This test environment does not guarantee a writable/mounted /data.
+		// If /data is absent we must keep /app fallback to avoid persistence failures.
+		if _, err := os.Stat("/data"); err == nil {
+			t.Skip("/data exists in this environment; cannot validate missing-/data fallback")
+		}
+		if got := containerDefaultConfigPath(); got != "/app/config.json" {
+			t.Fatalf("containerDefaultConfigPath() = %q, want %q", got, "/app/config.json")
+		}
+	})
+
+	t.Run("prefer /data when /data directory exists", func(t *testing.T) {
+		if _, err := os.Stat("/data"); err != nil {
+			t.Skip("/data does not exist in this environment")
+		}
+		if got := containerDefaultConfigPath(); got != "/data/config.json" {
+			t.Fatalf("containerDefaultConfigPath() = %q, want %q", got, "/data/config.json")
+		}
+	})
+}