Do not log an upload stopped by Ctrl-C as an error (closes #434)

Pressing Ctrl-C or sending SIGTERM during a backup logged the interrupted blob upload as an ERROR before the interrupted message. The upload step now skips that record when the backup's context has been cancelled, whatever error the storage returned. An upload that fails for any other reason is still logged.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -1861,7 +1861,11 @@ func (s *Scanner) uploadBlobIfNeeded(
 	err = s.storage.PutWithProgress(ctx, blobPath, blobWithReader.Reader,
 		finishedBlob.Compressed, progressCallback)
 	if err != nil {
-		log.Error("Failed to upload blob", "hash", finishedBlob.Hash, "error", err)
+		// Only an interrupt cancels ctx, and the user is told about it,
+		// so an upload it stopped is not logged as a storage failure.
+		if ctx.Err() == nil {
+			log.Error("Failed to upload blob", "hash", finishedBlob.Hash, "error", err)
+		}
 
 		return 0, false, fmt.Errorf("uploading blob to storage: %w", err)
 	}