fix(sni): prevent use-after-free and infinite retry in Host proxy retry

Store the retry timeout connection in a Host member and disconnect it in
~Host, so a Host destroyed within the retry window no longer invokes the
timeout callback on freed memory. Cap the retries (MAX_RETRIES) so a
watcher that never reappears no longer spins forever every 200ms; reset
the counter on success and when the watcher name vanishes.

include/modules/sni/host.hpp

@@ -43,6 +43,8 @@ class Host {
   std::size_t watcher_id_;
   GCancellable* cancellable_ = nullptr;
   SnWatcher* watcher_ = nullptr;
+  sigc::connection retry_connection_;
+  unsigned retry_count_ = 0;
   const Json::Value& config_;
   const Bar& bar_;
   const std::function<void(std::unique_ptr<Item>&)> on_add_;