Change FileProgress callback to channel-based progress

Replace callback-based progress reporting in Builder.AddFile with
channel-based FileHashProgress for consistency with EnumerateStatus
and ScanStatus patterns. Update scanner.go to use the new channel API.

internal/cli/gen.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/spf13/afero"
 	"github.com/urfave/cli/v2"
 	"sneak.berlin/go/mfer/internal/log"
 	"sneak.berlin/go/mfer/internal/scanner"
@@ -62,8 +63,15 @@ func (mfa *CLIApp) generateManifestOperation(ctx *cli.Context) error {
 
 	log.Debugf("enumerated %d files, %d bytes total", s.FileCount(), s.TotalBytes())
 
-	// Open output file
+	// Check if output file exists
 	outputPath := ctx.String("output")
+	if exists, _ := afero.Exists(mfa.Fs, outputPath); exists {
+		if !ctx.Bool("force") {
+			return fmt.Errorf("output file %s already exists (use --force to overwrite)", outputPath)
+		}
+	}
+
+	// Open output file
 	outFile, err := mfa.Fs.Create(outputPath)
 	if err != nil {
 		return fmt.Errorf("failed to create output file: %w", err)
@@ -76,10 +84,18 @@ func (mfa *CLIApp) generateManifestOperation(ctx *cli.Context) error {
 		scanProgress = make(chan scanner.ScanStatus, 1)
 		go func() {
 			for status := range scanProgress {
-				log.Progressf("Scanning: %d/%d files, %.1f MB/s",
-					status.ScannedFiles,
-					status.TotalFiles,
-					status.BytesPerSec/1e6)
+				if status.ETA > 0 {
+					log.Progressf("Scanning: %d/%d files, %.1f MB/s, ETA %s",
+						status.ScannedFiles,
+						status.TotalFiles,
+						status.BytesPerSec/1e6,
+						status.ETA.Round(time.Second))
+				} else {
+					log.Progressf("Scanning: %d/%d files, %.1f MB/s",
+						status.ScannedFiles,
+						status.TotalFiles,
+						status.BytesPerSec/1e6)
+				}
 			}
 			log.ProgressDone()
 		}()