A plain pipe into the compressor only reported the compressor's exit
status, so a failed dump still produced a small, valid archive. Add
pipeToFileKeepingExitStatus() to propagate the source's exit status in
POSIX sh and use it for all compressed dump commands.
- Delete the partial local backup file on any failed backup, not only
for ClickHouse, since untracked files are never removed by retention
- Wait up to 10s for locks when running VACUUM INTO for SQLite backups
- Refuse SQLite restores unless the file has a SQLite header, so an
empty backup no longer wipes the database
- Add tests for dump exit status handling and SQLite restore validation
