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

frontend/src/i18n/locales/en.ts

@@ -4633,8 +4633,12 @@ export default {
         title: 'Balance Low Notification',
         description: 'Send email notification when user balance falls below threshold',
         enabled: 'Enable Balance Low Notification',
+        thresholdType: 'Threshold Type',
+        typeFixed: 'Fixed Amount',
+        typePercentage: 'Percentage of Recharged',
         threshold: 'Default Threshold',
         thresholdHint: 'Used when user has not set a custom value',
+        percentageHint: 'Notify when balance falls below this percentage of total recharged amount',
         thresholdPlaceholder: 'Enter amount',
       },
       quotaNotify: {

frontend/src/i18n/locales/zh.ts

@@ -4797,8 +4797,12 @@ export default {
         title: '余额不足提醒',
         description: '当用户余额低于阈值时发送邮件提醒',
         enabled: '启用余额不足提醒',
-        threshold: '默认提醒阈值',
+        thresholdType: '阈值类型',
+        typeFixed: '固定金额',
+        typePercentage: '充值百分比',
+        threshold: '提醒阈值',
         thresholdHint: '用户未自定义时使用此值',
+        percentageHint: '当余额低于累计充值额的此百分比时提醒',
         thresholdPlaceholder: '输入金额',
       },
       quotaNotify: {