fix: remove double cleanShutdown call (closes #21)

The serve() method called cleanShutdown() after ctx.Done(), and the fx
OnStop hook also called cleanShutdown(). Remove the call in serve() so
shutdown happens exactly once via the fx lifecycle.

internal/server/server.go

@@ -109,7 +109,7 @@ func (s *Server) serve() int {
 		s.log.Info("signal received", "signal", sig.String())
 		if s.cancelFunc != nil {
 			// cancelling the main context will trigger a clean
-			// shutdown.
+			// shutdown via the fx OnStop hook.
 			s.cancelFunc()
 		}
 	}()
@@ -117,7 +117,8 @@ func (s *Server) serve() int {
 	go s.serveUntilShutdown()
 
 	<-s.ctx.Done()
-	s.cleanShutdown()
+	// Shutdown is handled by the fx OnStop hook (cleanShutdown).
+	// Do not call cleanShutdown() here to avoid a double invocation.
 	return s.exitCode
 }