CLI: Add "photoprism dl --format-sort" flag and dl-method env variable

Signed-off-by: Michael Mayer <michael@photoprism.app>

internal/commands/download_method.go

@@ -0,0 +1,41 @@
+package commands
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+const downloadMethodEnv = "PHOTOPRISM_DL_METHOD"
+
+// resolveDownloadMethod normalizes the download method by honoring the explicit flag
+// value first, then the environment variable PHOTOPRISM_DL_METHOD, and finally
+// defaulting to "pipe". It returns the resolved method, a boolean indicating whether
+// the value originated from the environment, or an error when the input is invalid.
+func resolveDownloadMethod(flagValue string) (string, bool, error) {
+	trimmed := strings.TrimSpace(flagValue)
+	method := strings.ToLower(trimmed)
+	fromEnv := false
+
+	if method == "" {
+		envValue := strings.TrimSpace(os.Getenv(downloadMethodEnv))
+		if envValue != "" {
+			method = strings.ToLower(envValue)
+			trimmed = envValue
+			fromEnv = true
+		}
+	}
+
+	if method == "" {
+		return "pipe", false, nil
+	}
+
+	if method != "pipe" && method != "file" {
+		if fromEnv {
+			return "", true, fmt.Errorf("invalid %s value: %s (expected 'pipe' or 'file')", downloadMethodEnv, trimmed)
+		}
+		return "", false, fmt.Errorf("invalid download method: %s (expected 'pipe' or 'file')", trimmed)
+	}
+
+	return method, fromEnv, nil
+}