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/user.go

@@ -31,9 +31,11 @@ type User struct {
 	TotpEnabledAt       *time.Time // TOTP 启用时间
 
 	// 余额不足通知
-	BalanceNotifyEnabled     bool
-	BalanceNotifyThreshold   *float64
-	BalanceNotifyExtraEmails []string
+	BalanceNotifyEnabled       bool
+	BalanceNotifyThresholdType string   // "fixed" (default) | "percentage"
+	BalanceNotifyThreshold     *float64
+	BalanceNotifyExtraEmails   []string
+	TotalRecharged             float64
 
 	APIKeys       []APIKey
 	Subscriptions []UserSubscription