feat(auth): 实现 TOTP 双因素认证功能

新增功能：
- 支持 Google Authenticator 等应用进行 TOTP 二次验证
- 用户可在个人设置中启用/禁用 2FA
- 登录时支持 TOTP 验证流程
- 管理后台可全局开关 TOTP 功能

安全增强：
- TOTP 密钥使用 AES-256-GCM 加密存储
- 添加 TOTP_ENCRYPTION_KEY 配置项，必须手动配置才能启用功能
- 防止服务重启导致加密密钥变更使用户无法登录
- 验证失败次数限制，防止暴力破解

配置说明：
- Docker 部署：在 .env 中设置 TOTP_ENCRYPTION_KEY
- 非 Docker 部署：在 config.yaml 中设置 totp.encryption_key
- 生成密钥命令：openssl rand -hex 32

backend/ent/user/user.go

@@ -37,6 +37,12 @@ const (
 	FieldUsername = "username"
 	// FieldNotes holds the string denoting the notes field in the database.
 	FieldNotes = "notes"
+	// FieldTotpSecretEncrypted holds the string denoting the totp_secret_encrypted field in the database.
+	FieldTotpSecretEncrypted = "totp_secret_encrypted"
+	// FieldTotpEnabled holds the string denoting the totp_enabled field in the database.
+	FieldTotpEnabled = "totp_enabled"
+	// FieldTotpEnabledAt holds the string denoting the totp_enabled_at field in the database.
+	FieldTotpEnabledAt = "totp_enabled_at"
 	// EdgeAPIKeys holds the string denoting the api_keys edge name in mutations.
 	EdgeAPIKeys = "api_keys"
 	// EdgeRedeemCodes holds the string denoting the redeem_codes edge name in mutations.
@@ -134,6 +140,9 @@ var Columns = []string{
 	FieldStatus,
 	FieldUsername,
 	FieldNotes,
+	FieldTotpSecretEncrypted,
+	FieldTotpEnabled,
+	FieldTotpEnabledAt,
 }
 
 var (
@@ -188,6 +197,8 @@ var (
 	UsernameValidator func(string) error
 	// DefaultNotes holds the default value on creation for the "notes" field.
 	DefaultNotes string
+	// DefaultTotpEnabled holds the default value on creation for the "totp_enabled" field.
+	DefaultTotpEnabled bool
 )
 
 // OrderOption defines the ordering options for the User queries.
@@ -253,6 +264,21 @@ func ByNotes(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldNotes, opts...).ToFunc()
 }
 
+// ByTotpSecretEncrypted orders the results by the totp_secret_encrypted field.
+func ByTotpSecretEncrypted(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldTotpSecretEncrypted, opts...).ToFunc()
+}
+
+// ByTotpEnabled orders the results by the totp_enabled field.
+func ByTotpEnabled(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldTotpEnabled, opts...).ToFunc()
+}
+
+// ByTotpEnabledAt orders the results by the totp_enabled_at field.
+func ByTotpEnabledAt(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldTotpEnabledAt, opts...).ToFunc()
+}
+
 // ByAPIKeysCount orders the results by api_keys count.
 func ByAPIKeysCount(opts ...sql.OrderTermOption) OrderOption {
 	return func(s *sql.Selector) {