feat(notify): add percentage threshold type for balance low notification

- Add threshold_type field (fixed/percentage) to system and user settings
- Add total_recharged field to users table, auto-incremented on balance credit
- Percentage mode: effective threshold = total_recharged × percentage / 100
- User-level threshold_type inherits from system default when not set
- Update admin settings UI with radio selector (fixed amount / percentage)
- Migration: 102_add_balance_notify_threshold_type.sql

backend/internal/service/balance_notify_service.go

@@ -47,30 +47,21 @@ func (s *BalanceNotifyService) CheckBalanceAfterDeduction(ctx context.Context, u
 	if user == nil || s.emailService == nil || s.settingRepo == nil {
 		return
 	}
-
-	// Check user-level switch
 	if !user.BalanceNotifyEnabled {
 		return
 	}
 
-	// Check global switch
-	globalEnabled, threshold := s.getBalanceNotifyConfig(ctx)
+	globalEnabled, globalThresholdType, globalThresholdValue := s.getBalanceNotifyConfig(ctx)
 	if !globalEnabled {
 		return
 	}
 
-	// User custom threshold overrides system default
-	if user.BalanceNotifyThreshold != nil {
-		threshold = *user.BalanceNotifyThreshold
-	}
-
+	threshold := s.resolveEffectiveThreshold(user, globalThresholdType, globalThresholdValue)
 	if threshold <= 0 {
 		return
 	}
 
 	newBalance := oldBalance - cost
-
-	// Only notify on first crossing
 	if oldBalance >= threshold && newBalance < threshold {
 		siteName := s.getSiteName(ctx)
 		recipients := s.collectBalanceNotifyRecipients(user)
@@ -85,6 +76,30 @@ func (s *BalanceNotifyService) CheckBalanceAfterDeduction(ctx context.Context, u
 	}
 }
 
+// resolveEffectiveThreshold computes the actual USD threshold based on type and user settings.
+func (s *BalanceNotifyService) resolveEffectiveThreshold(user *User, globalType string, globalValue float64) float64 {
+	// User-level override takes full precedence
+	if user.BalanceNotifyThreshold != nil {
+		thresholdType := user.BalanceNotifyThresholdType
+		if thresholdType == "" {
+			thresholdType = globalType
+		}
+		return computeThreshold(thresholdType, *user.BalanceNotifyThreshold, user.TotalRecharged)
+	}
+	return computeThreshold(globalType, globalValue, user.TotalRecharged)
+}
+
+// computeThreshold converts a threshold value to USD based on type.
+func computeThreshold(thresholdType string, value, totalRecharged float64) float64 {
+	if thresholdType == ThresholdTypePercentage {
+		if totalRecharged <= 0 {
+			return 0 // no recharge history → skip percentage check
+		}
+		return totalRecharged * value / 100
+	}
+	return value // fixed USD amount
+}
+
 // quotaDim describes one quota dimension for notification checking.
 type quotaDim struct {
 	name      string
@@ -139,13 +154,21 @@ func (s *BalanceNotifyService) asyncSendQuotaAlert(adminEmails []string, account
 }
 
 // getBalanceNotifyConfig reads global balance notification settings.
-func (s *BalanceNotifyService) getBalanceNotifyConfig(ctx context.Context) (enabled bool, threshold float64) {
-	keys := []string{SettingKeyBalanceLowNotifyEnabled, SettingKeyBalanceLowNotifyThreshold}
+func (s *BalanceNotifyService) getBalanceNotifyConfig(ctx context.Context) (enabled bool, thresholdType string, threshold float64) {
+	keys := []string{
+		SettingKeyBalanceLowNotifyEnabled,
+		SettingKeyBalanceLowNotifyThresholdType,
+		SettingKeyBalanceLowNotifyThreshold,
+	}
 	settings, err := s.settingRepo.GetMultiple(ctx, keys)
 	if err != nil {
-		return false, 0
+		return false, ThresholdTypeFixed, 0
 	}
 	enabled = settings[SettingKeyBalanceLowNotifyEnabled] == "true"
+	thresholdType = settings[SettingKeyBalanceLowNotifyThresholdType]
+	if thresholdType == "" {
+		thresholdType = ThresholdTypeFixed
+	}
 	if v := settings[SettingKeyBalanceLowNotifyThreshold]; v != "" {
 		if f, err := strconv.ParseFloat(v, 64); err == nil {
 			threshold = f