refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/auth/constants.go

@@ -0,0 +1,40 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package auth
+
+import (
+	"time"
+)
+
+// Session and Context Keys
+const (
+	UserNameKey     = "username"
+	UserIDKey       = "user_id"
+	UserObjKey      = "user_obj"
+	TokenAuthKey    = "token_auth"          // 标记当前请求是否通过 Access Token 鉴权
+	TokenAdminKey   = "token_admin"         // Access Token 本身是否具有管理员权限
+	SessionTokenKey = "oauth_session_token" //nolint:gosec // false positive: this is a session key, not hardcoded credentials
+	PasswordHashKey = "password_hash"
+	SystemUsername  = "system"
+)
+
+// OAuth State Cache Keys and Expirations
+const (
+	OAuthStateCacheKeyFormat     = "oauth:state:%s"
+	OAuthStateCacheKeyExpiration = 10 * time.Minute
+	oauthStateLimitKeyFormat     = "oauth:state:limit:%s"
+	oauthStateLimitMax           = 10
+)
+
+// OAuth Purpose Constants
+const (
+	OAuthPurposeLogin = "login"
+	OAuthPurposeBind  = "bind"
+)
+
+// Auth Source Types
+const (
+	AuthSourceTypeOIDC = "oidc"
+)