feat(notify): add platform/ID to quota alert email, add recharge URL to balance alert

- Quota alert email now shows account ID and platform
- Balance low email includes a "Top Up Now" button when recharge URL is configured
- New setting: balance_low_notify_recharge_url in admin settings

backend/internal/service/settings_view.go

@@ -108,8 +108,9 @@ type SystemSettings struct {
 	EnableCCHSigning             bool // 是否对 billing header cch 进行签名（默认 false）
 
 	// Balance low notification
-	BalanceLowNotifyEnabled   bool
-	BalanceLowNotifyThreshold float64
+	BalanceLowNotifyEnabled    bool
+	BalanceLowNotifyThreshold  float64
+	BalanceLowNotifyRechargeURL string
 
 	// Account quota notification
 	AccountQuotaNotifyEnabled bool
@@ -157,6 +158,7 @@ type PublicSettings struct {
 	BalanceLowNotifyEnabled          bool
 	AccountQuotaNotifyEnabled        bool
 	BalanceLowNotifyThreshold        float64
+	BalanceLowNotifyRechargeURL      string
 }
 
 // StreamTimeoutSettings 流超时处理配置（仅控制超时后的处理方式，超时判定由网关配置控制）