Pass remote rsync and source-root preflight checks as a single quoted
shell command to SSH so the remote shell evaluates command -v and test
expressions reliably.
Refs #45
Add an on-demand host preflight action that verifies SSH reachability,
remote rsync availability, and remote source root access.
Persist the latest preflight result on the host config, render it in Django,
and block real backups when the last remote preflight failed.
Introduce a host preflight layer that separates dry-run blockers from real backup blockers.
Show the effective per-host backup configuration in Django before queueing a run.
Block real backup queueing when failed host checks remain, while still allowing dry-runs
when only local storage preparation is missing.
