fix: Caddy health check must hit /config/ not /

GET http://cell-caddy:2019/ returns 404 because Caddy's admin API has no
root handler.  The health monitor interpreted every response as a failure,
restarted Caddy every 3 minutes, and prevented ACME from ever completing.

/config/ returns 200 + the running config JSON whenever Caddy is up and
serving — that is the correct liveness indicator.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_caddy_manager.py

@@ -241,8 +241,8 @@ class TestHealthCheck(unittest.TestCase):
             mock_get.return_value = MagicMock(status_code=200)
             self.assertTrue(mgr.check_caddy_health())
         mock_get.assert_called_once()
-        # URL must be the admin API root
-        self.assertIn('cell-caddy:2019', mock_get.call_args[0][0])
+        # Must hit /config/ — not the root which returns 404
+        self.assertIn('/config/', mock_get.call_args[0][0])
 
     def test_returns_false_on_connection_error(self):
         mgr = _mgr()