Count a file a backup could not store as failed (closes #280)

A file that phase 1 of a backup counted and phase 2 could not open,
because it was unreadable under --skip-errors or removed in between,
was added to the unchanged count while its size stayed in
BytesScanned. The summary showed it as unchanged with its bytes backed
up, and the snapshots row's file_count and total_size included it.

The scanner now counts such a file in FilesFailed and takes its size
out of BytesScanned. The summary's files line adds "N failed", and
file_count leaves the file out.

A directory phase 2 cannot record is not counted as failed, since
phase 1 counts no directories; that case has no test.

Model: opus-5-5

internal/snapshot/snapshot.go

@@ -892,7 +892,7 @@ func (sm *SnapshotManager) getFileSize(path string) int64 {
 // BackupStats contains statistics from a backup operation
 type BackupStats struct {
 	FilesScanned  int
-	TotalSize     int64 // Total size of all files examined
+	TotalSize     int64 // Total size of the files in the snapshot
 	ChunksCreated int
 	BlobsCreated  int
 	BytesUploaded int64