feat(gateway): add retry_on_refused for pymsg MsgClient (v0.6.1)

- Add MsgConnectionRefusedException to distinguish ECONNREFUSED from other
  connection errors; openConnection() now throws it when connect() returns
  ECONNREFUSED.
- MsgClient::call() adds retry_on_refused/retry_interval parameters (defaults
  false/1.0s), matching Python pymsg.MsgClient.call(retry_on_refused=True).
- Retry loop: only retries on ECONNREFUSED; other exceptions (timeout,
  connection reset, business errors) propagate immediately. Throttled logging:
  first failure prints immediately, subsequent logs every 5 seconds.
- DbC::call_msg() enables retry_on_refused=true (db is a hard dependency;
  gateway blocks waiting for db on startup instead of crashing).
- LogC::_send() explicitly passes retry_on_refused=false (fail-safe; never
  block the background log thread when db is unavailable).
- captcha/smtp auxiliary services keep default retry_on_refused=false
  (fail-fast; blocking in request threads would exhaust the thread pool).
- Update README with pymsg protocol documentation, downstream retry strategy
  table, and v0.6.1 changelog entry.

gateway/inc/LogC.hpp

@@ -196,7 +196,8 @@ private:
 				payload["values"] = std::move(values);
 
 				const std::string resp = client.call(
-						json::serialize(payload), _TIMEOUT);
+						json::serialize(payload), _TIMEOUT,
+						/*retry_on_refused=*/false);  // 日志是 fail-safe，db 不可用时静默丢弃，绝不阻塞重试
 				auto obj = json::parse(resp).as_object();
 				if (!obj.at("success").as_bool()) {
 					std::string err = "unknown";