saves responses now include one device_syncs entry per device that has
synced a save, not just the caller's, so clients can tell which devices
hold a save. is_current is computed per entry and the caller's own entry
is ordered first for backward compatibility.
add a saves.origin_device_id column (migration 0081) recording the
device that created a save, set on initial upload only, surfaced as
origin_device_id on the save schema.
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