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/profile.go

@@ -7,7 +7,7 @@ import (
 // Profile Minecraft 档案模型
 // @Description Minecraft角色档案数据模型
 type Profile struct {
-	UUID          string     `gorm:"column:uuid;type:varchar(36);primaryKey" json:"uuid"`
+	UUID          string     `gorm:"column:uuid;type:varchar(32);primaryKey" json:"uuid"`
 	UserID        int64      `gorm:"column:user_id;not null;index:idx_profiles_user_created,priority:1" json:"user_id"`
 	Name          string     `gorm:"column:name;type:varchar(16);not null;uniqueIndex:idx_profiles_name" json:"name"` // Minecraft 角色名
 	SkinID        *int64     `gorm:"column:skin_id;type:bigint;index:idx_profiles_skin_id" json:"skin_id,omitempty"`