Add a health check for the daemon and RPC endpoint (#34)

* 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.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

healthcheck.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+"""Docker HEALTHCHECK entry point.
+
+Exits 0 if the seafile daemon and its RPC socket both respond, 1 otherwise.
+Runs as a separate, short-lived process on every check, so it needs no
+credentials and touches no state the main process owns.
+"""
+
+import sys
+
+from dsc import const
+from dsc.client import SeafileClient
+
+
+def main() -> int:
+    client = SeafileClient(host="unused", user="unused", app_dir=const.DEFAULT_APP_DIR)
+    return 0 if client.is_healthy() else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())