feat(yggdrasil): implement standard error responses and UUID format improvements

- Add YggdrasilErrorResponse struct and standard error codes for protocol compliance
- Change UUID storage from varchar(36) to varchar(32) for unsigned format
- Add utility functions: GenerateUUID, FormatUUIDToNoDash, RandomHex
- Support unsigned query parameter in GetProfileByUUID endpoint
- Improve refresh token response with available profiles list
- Fix key pair retrieval to use correct database column (rsa_private_key)
- Update UUID validator to accept both 32-char and 36-char formats
- Add SignStringWithProfileRSA method for profile-specific signing
- Fix profile assignment validation in refresh token flow

internal/model/client.go

@@ -5,10 +5,10 @@ import "time"
 // Client 客户端实体，用于管理Token版本
 // @Description Yggdrasil客户端Token管理数据
 type Client struct {
-	UUID        string    `gorm:"column:uuid;type:varchar(36);primaryKey" json:"uuid"`                                         // Client UUID
+	UUID        string    `gorm:"column:uuid;type:varchar(32);primaryKey" json:"uuid"`                                         // Client UUID
 	ClientToken string    `gorm:"column:client_token;type:varchar(64);not null;uniqueIndex" json:"client_token"`               // 客户端Token
 	UserID      int64     `gorm:"column:user_id;not null;index:idx_clients_user_id" json:"user_id"`                            // 用户ID
-	ProfileID   string    `gorm:"column:profile_id;type:varchar(36);index:idx_clients_profile_id" json:"profile_id,omitempty"` // 选中的Profile
+	ProfileID   string    `gorm:"column:profile_id;type:varchar(32);index:idx_clients_profile_id" json:"profile_id,omitempty"` // 选中的Profile
 	Version     int       `gorm:"column:version;not null;default:0;index:idx_clients_version" json:"version"`                  // 版本号
 	CreatedAt   time.Time `gorm:"column:created_at;type:timestamp;not null;default:CURRENT_TIMESTAMP" json:"created_at"`
 	UpdatedAt   time.Time `gorm:"column:updated_at;type:timestamp;not null;default:CURRENT_TIMESTAMP" json:"updated_at"`