Snapshot admin surface. Mounted as a unit so that
/admin/snapshot/{save,load} always have a configured default
path: an operator who set --snapshot-path is the one paying the
cost of the route’s existence, and they reasonably expect the
path to be resolved automatically when no path field is sent in
the request body.
Request body for POST /admin/snapshot/{save,load}. The body is
optional; when it is absent (or an empty JSON object) the server uses
the path configured in AdminConfig.
Body for POST /admin/wal/truncate. Operators supply the LSN past
which sealed segments may be deleted; the WAL truncates everything
at or below that point. Active and tombstone segments are always
retained.