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>

dsc/client.py

@@ -165,6 +165,22 @@ class SeafileClient:
         self.__wait_for_daemon(False, const.DAEMON_STOP_TIMEOUT, "stopping")
         _lg.info("Seafile daemon is stopped")
 
+    def is_healthy(self) -> bool:
+        """
+        Report whether the daemon process and its RPC socket both respond.
+
+        Used by the container's HEALTHCHECK. Neither check needs the Seafile
+        server, so this works without a token or password.
+        """
+        if not self.daemon_ready:
+            return False
+        try:
+            self.rpc.get_repo_list(-1, -1)
+        except Exception as err:
+            _lg.warning("RPC endpoint is not responding: %s", err)
+            return False
+        return True
+
     def get_library_id(self, library) -> Optional[str]:
         """
         Resolve a requested name or ID to the one library ID it names.