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/repository/email_cache.go

@@ -11,6 +11,7 @@ import (
 
 const (
 	verifyCodeKeyPrefix          = "verify_code:"
+	notifyVerifyKeyPrefix        = "notify_verify:"
 	passwordResetKeyPrefix       = "password_reset:"
 	passwordResetSentAtKeyPrefix = "password_reset_sent:"
 )
@@ -20,6 +21,11 @@ func verifyCodeKey(email string) string {
 	return verifyCodeKeyPrefix + email
 }
 
+// notifyVerifyKey generates the Redis key for notify email verification code.
+func notifyVerifyKey(email string) string {
+	return notifyVerifyKeyPrefix + email
+}
+
 // passwordResetKey generates the Redis key for password reset token.
 func passwordResetKey(email string) string {
 	return passwordResetKeyPrefix + email
@@ -106,3 +112,32 @@ func (c *emailCache) SetPasswordResetEmailCooldown(ctx context.Context, email st
 	key := passwordResetSentAtKey(email)
 	return c.rdb.Set(ctx, key, "1", ttl).Err()
 }
+
+// Notify email verification code methods
+
+func (c *emailCache) GetNotifyVerifyCode(ctx context.Context, email string) (*service.VerificationCodeData, error) {
+	key := notifyVerifyKey(email)
+	val, err := c.rdb.Get(ctx, key).Result()
+	if err != nil {
+		return nil, err
+	}
+	var data service.VerificationCodeData
+	if err := json.Unmarshal([]byte(val), &data); err != nil {
+		return nil, err
+	}
+	return &data, nil
+}
+
+func (c *emailCache) SetNotifyVerifyCode(ctx context.Context, email string, data *service.VerificationCodeData, ttl time.Duration) error {
+	key := notifyVerifyKey(email)
+	val, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	return c.rdb.Set(ctx, key, val, ttl).Err()
+}
+
+func (c *emailCache) DeleteNotifyVerifyCode(ctx context.Context, email string) error {
+	key := notifyVerifyKey(email)
+	return c.rdb.Del(ctx, key).Err()
+}