* Add a health check for the daemon and RPC endpoint
SeafileClient.is_healthy() checks seaf-cli status and makes a real RPC
call (get_repo_list) against the daemon's Unix socket - neither needs
the Seafile server, so it works without a token or password.
healthcheck.py exposes it as a plain script for Docker's HEALTHCHECK.
Each check is a fresh process spawned by the Docker daemon as root,
unlike the long-lived start.py process, so it drops to the seafile
user with runuser before touching the RPC socket - the one place that
mechanism still belongs after the earlier in-process privilege drop.
Verified against the real image: healthcheck.py exits 0 once the real
daemon is up (through the actual main() entrypoint, not a test
fixture) and nonzero when no daemon has been started.
* Assert the return value in the daemon-down health check test
Copilot review: the test checked the RPC call was skipped but not
what is_healthy() actually returned in that case.
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Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
