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>

tests/test_healthcheck.py

@@ -0,0 +1,40 @@
+"""The container HEALTHCHECK must reflect both the daemon and its RPC socket."""
+
+import pytest
+
+from dsc.client import SeafileClient
+
+
+@pytest.fixture
+def client():
+    return SeafileClient("seafile.example", "user", "pw", "/dsc", token="tok-en")
+
+
+def test_healthy_when_daemon_and_rpc_both_respond(client, monkeypatch):
+    monkeypatch.setattr(type(client), "daemon_ready", True)
+    monkeypatch.setattr(client.rpc, "get_repo_list", lambda a, b: [])
+    assert client.is_healthy() is True
+
+
+def test_unhealthy_when_the_daemon_is_not_ready(client, monkeypatch):
+    monkeypatch.setattr(type(client), "daemon_ready", False)
+    assert client.is_healthy() is False
+
+
+def test_unhealthy_when_the_rpc_socket_raises(client, monkeypatch):
+    monkeypatch.setattr(type(client), "daemon_ready", True)
+
+    def broken(a, b):
+        raise ConnectionError("socket gone")
+
+    monkeypatch.setattr(client.rpc, "get_repo_list", broken)
+    assert client.is_healthy() is False
+
+
+def test_rpc_is_not_checked_when_the_daemon_is_already_down(client, monkeypatch):
+    """A dead daemon means a dead socket too; no need to touch it."""
+    monkeypatch.setattr(type(client), "daemon_ready", False)
+    called = []
+    monkeypatch.setattr(client.rpc, "get_repo_list", lambda a, b: called.append(1))
+    assert client.is_healthy() is False
+    assert not called