snapshot list summed the chunks a snapshot reaches through its file rows
in the local index. A file's row is shared by every snapshot that lists
it and rewritten when the file changes, so backing up a changed file
changed every older snapshot's UNCOMPRESSED SIZE.
The backup now takes that total when it records the snapshot's stats,
into a new snapshots column, uncompressed_chunk_size, and snapshot list
reads the column, as it already does for new_chunk_size.
Model: opus-5-5
