Print upload start line and 15s heartbeat during blob upload

Long-running uploads (multi-GB blobs over slow links) previously
produced silence between the start of the upload and the "Blob
stored" line at the end. Now we print:

  Uploading blob: <hash> (<size>)

before the upload starts, and a heartbeat line at most every 15s:

  uploading <hash>: <done>/<total> (NN%), <speed>/sec, <elapsed> elapsed, ETA <eta>

This gives the user visible progress on large uploads, especially
over SMB or remote storage where 10+ second stalls are normal.

internal/snapshot/scanner.go

@@ -1169,7 +1169,11 @@ func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobW
 		return true, nil
 	}
 
-	progressCallback := s.makeUploadProgressCallback(ctx, finishedBlob)
+	_, _ = fmt.Fprintf(s.output, "Uploading blob: %s (%s)\n",
+		finishedBlob.Hash[:12]+"...",
+		humanize.Bytes(uint64(finishedBlob.Compressed)))
+
+	progressCallback := s.makeUploadProgressCallback(ctx, finishedBlob, startTime)
 
 	if err := s.storage.PutWithProgress(ctx, blobPath, blobWithReader.Reader, finishedBlob.Compressed, progressCallback); err != nil {
 		log.Error("Failed to upload blob", "hash", finishedBlob.Hash, "error", err)
@@ -1201,10 +1205,14 @@ func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobW
 	return false, nil
 }
 
-// makeUploadProgressCallback creates a progress callback for blob uploads
+// makeUploadProgressCallback creates a progress callback for blob uploads.
-func (s *Scanner) makeUploadProgressCallback(ctx context.Context, finishedBlob *blob.FinishedBlob) func(int64) error {
+// It updates the live progress reporter ~twice/sec for ETAs and prints a
+// human-readable status line to s.output at most every 15 seconds.
+func (s *Scanner) makeUploadProgressCallback(ctx context.Context, finishedBlob *blob.FinishedBlob, uploadStart time.Time) func(int64) error {
 	lastProgressTime := time.Now()
 	lastProgressBytes := int64(0)
+	lastStdoutTime := time.Now()
+	const stdoutInterval = 15 * time.Second
 
 	return func(uploaded int64) error {
 		now := time.Now()
@@ -1218,6 +1226,27 @@ func (s *Scanner) makeUploadProgressCallback(ctx context.Context, finishedBlob *
 			lastProgressTime = now
 			lastProgressBytes = uploaded
 		}
+
+		// Periodic stdout status line so the user knows the upload is alive.
+		if now.Sub(lastStdoutTime) >= stdoutInterval {
+			totalElapsed := now.Sub(uploadStart)
+			pct := float64(uploaded) / float64(finishedBlob.Compressed) * 100
+			avgSpeed := float64(uploaded) / totalElapsed.Seconds()
+			var eta time.Duration
+			if avgSpeed > 0 {
+				eta = time.Duration(float64(finishedBlob.Compressed-uploaded)/avgSpeed) * time.Second
+			}
+			_, _ = fmt.Fprintf(s.output, "  uploading %s: %s/%s (%.0f%%), %s/sec, %s elapsed, ETA %s\n",
+				finishedBlob.Hash[:12]+"...",
+				humanize.Bytes(uint64(uploaded)),
+				humanize.Bytes(uint64(finishedBlob.Compressed)),
+				pct,
+				humanize.Bytes(uint64(avgSpeed)),
+				totalElapsed.Round(time.Second),
+				eta.Round(time.Second))
+			lastStdoutTime = now
+		}
+
 		select {
 		case <-ctx.Done():
 			return ctx.Err()