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/src/main.cpp

@@ -28,7 +28,7 @@ using namespace qing;
 namespace json = boost::json;
 namespace po = boost::program_options;
 
-constexpr const char *_VER = "0.6.0";
+constexpr const char *_VER = "0.6.1";
 
 //////////////////////////////////
 #include <sstream>