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

backend/internal/service/auth_service_register_test.go

@@ -87,6 +87,18 @@ func (s *emailCacheStub) DeleteVerificationCode(ctx context.Context, email strin
 	return nil
 }
 
+func (s *emailCacheStub) GetNotifyVerifyCode(ctx context.Context, email string) (*VerificationCodeData, error) {
+	return nil, nil
+}
+
+func (s *emailCacheStub) SetNotifyVerifyCode(ctx context.Context, email string, data *VerificationCodeData, ttl time.Duration) error {
+	return nil
+}
+
+func (s *emailCacheStub) DeleteNotifyVerifyCode(ctx context.Context, email string) error {
+	return nil
+}
+
 func (s *emailCacheStub) GetPasswordResetToken(ctx context.Context, email string) (*PasswordResetTokenData, error) {
 	return nil, nil
 }