Bind serve's socket before writing client.json

A serve that lost the port race had already signed in, so client.json
named a port another engine held and every client talked to that one.
The socket is now bound first and handed to uvicorn.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QgG6TJw5qBZzaLghQSRVrq

backend/tests/test_cli.py

@@ -935,6 +935,38 @@ def test_serve_moves_off_a_port_that_is_taken() -> None:
     assert DEFAULT_PORT == 8000
 
 
+def test_a_port_that_cannot_be_bound_writes_no_credential(tmp_path, monkeypatch):
+    """The token is written once the socket is ours, not on the way to it.
+
+    Written first, a failed start left `client.json` naming a port another
+    project's engine holds — and every client here then talked to that one.
+    """
+    import socket
+    from types import SimpleNamespace
+
+    from fluksio import cli
+    from fluksio.core import bootstrap
+
+    admin = SimpleNamespace(id="admin", email="admin@example.com")
+    monkeypatch.setattr(cli, "_prepare", lambda data_dir: None)
+    monkeypatch.setattr(bootstrap, "ensure_superuser", lambda *a, **k: (admin, None))
+    # uvicorn's logging config would otherwise outlive this test.
+    monkeypatch.setattr(cli, "_log_config", lambda level: None)
+
+    data_dir = tmp_path / ".fluksio"
+    with socket.socket() as held:
+        held.bind(("127.0.0.1", 0))
+        held.listen(1)
+        taken = held.getsockname()[1]
+        args = cli._parser().parse_args(
+            ["serve", "--plain", "--port", str(taken), "--data-dir", str(data_dir)]
+        )
+        assert cli.cmd_serve(args) == 1
+
+    assert not (data_dir / "client.json").exists()
+    assert not (data_dir / cli.PIDFILE).exists()
+
+
 def test_a_duration_reads_the_way_an_age_does() -> None:
     """One notation for both, rather than one per place that printed one."""
     from fluksio.sdk.cli import _dur