feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

frontend/src/api/admin/settings.ts

@@ -134,6 +134,11 @@ export interface SystemSettings {
   payment_cancel_rate_limit_window: number
   payment_cancel_rate_limit_unit: string
   payment_cancel_rate_limit_window_mode: string
+
+  // Balance & quota notification
+  balance_low_notify_enabled: boolean
+  balance_low_notify_threshold: number
+  account_quota_notify_emails: string[]
 }
 
 export interface UpdateSettingsRequest {
@@ -233,6 +238,10 @@ export interface UpdateSettingsRequest {
   payment_cancel_rate_limit_window?: number
   payment_cancel_rate_limit_unit?: string
   payment_cancel_rate_limit_window_mode?: string
+  // Balance & quota notification
+  balance_low_notify_enabled?: boolean
+  balance_low_notify_threshold?: number
+  account_quota_notify_emails?: string[]
 }
 
 /**

frontend/src/api/user.ts

@@ -22,6 +22,9 @@ export async function getProfile(): Promise<User> {
  */
 export async function updateProfile(profile: {
   username?: string
+  balance_notify_enabled?: boolean
+  balance_notify_threshold?: number | null
+  balance_notify_extra_emails?: string[]
 }): Promise<User> {
   const { data } = await apiClient.put<User>('/user', profile)
   return data
@@ -45,10 +48,38 @@ export async function changePassword(
   return data
 }
 
+/**
+ * Send verification code for adding a notify email
+ * @param email - Email address to verify
+ */
+export async function sendNotifyEmailCode(email: string): Promise<void> {
+  await apiClient.post('/user/notify-email/send-code', { email })
+}
+
+/**
+ * Verify and add a notify email
+ * @param email - Email address to add
+ * @param code - Verification code
+ */
+export async function verifyNotifyEmail(email: string, code: string): Promise<void> {
+  await apiClient.post('/user/notify-email/verify', { email, code })
+}
+
+/**
+ * Remove a notify email
+ * @param email - Email address to remove
+ */
+export async function removeNotifyEmail(email: string): Promise<void> {
+  await apiClient.delete('/user/notify-email', { data: { email } })
+}
+
 export const userAPI = {
   getProfile,
   updateProfile,
-  changePassword
+  changePassword,
+  sendNotifyEmailCode,
+  verifyNotifyEmail,
+  removeNotifyEmail
 }
 
 export default userAPI