refactor(backend): rename OpenFlare directory to lowercase openflare

backend/openflare/plugins/server/domain/observability/errs.go

@@ -0,0 +1,9 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package observability defines shared error messages for observability operations.
+package observability
+
+const (
+	errInvalidStatusCode = "status_code 必须为 100-599 之间的整数"
+)