feat: add affiliate invite rebate flow and admin rebate-rate setting

backend/internal/service/domain_constants.go

@@ -18,6 +18,13 @@ const (
 	RoleUser  = domain.RoleUser
 )
 
+// Affiliate rebate settings
+const (
+	AffiliateRebateRateDefault = 20.0
+	AffiliateRebateRateMin     = 0.0
+	AffiliateRebateRateMax     = 100.0
+)
+
 // Platform constants
 const (
 	PlatformAnthropic   = domain.PlatformAnthropic
@@ -87,6 +94,7 @@ const (
 	SettingKeyPasswordResetEnabled             = "password_reset_enabled"              // 是否启用忘记密码功能（需要先开启邮件验证）
 	SettingKeyFrontendURL                      = "frontend_url"                        // 前端基础URL，用于生成邮件中的重置密码链接
 	SettingKeyInvitationCodeEnabled            = "invitation_code_enabled"             // 是否启用邀请码注册
+	SettingKeyAffiliateRebateRate              = "affiliate_rebate_rate"               // 邀请返利比例（百分比，0-100）
 
 	// 邮件服务设置
 	SettingKeySMTPHost     = "smtp_host"      // SMTP服务器地址