feat(api): GET/PUT /api/config serving app + env configuration

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/cmd/geniusrund/main.go

@@ -66,6 +66,10 @@ func main() {
 		FrontendURL: cfg.FrontendURL,
 	})
 
+	for _, e := range cfg.DisplayEnv() {
+		server.EnvVars = append(server.EnvVars, api.EnvVar{Name: e.Name, Value: e.Value})
+	}
+
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()