Option A (validation):
- handlers_syncpairs.go: reject create/update when both SourceMachineID
and DestMachineID are set; clear error message explains the constraint
- engine.go: detect 'both remote' rsync error at runtime and surface it
as error_code=remote_to_remote_unsupported with a human-readable message
Option B (remote-to-remote support):
- rsync_runner.go: add RunRemote() method that SSHs to the source machine
and runs rsync locally there (src=local path, dst=user@host:/path),
streaming output back through the onLine callback
- engine.go: when both srcMachine and dstMachine are non-nil, use
RunRemote() instead of Run(), SSHing to srcMachine and running rsync
from there. Also wake dstMachine via WoL when both sides are remote.
darroyo
d7e6d64967