feat: add registration abuse prevention

- Silently block verification code for IPs with 2+ registered accounts
- Silently block Gmail alias emails (containing + or . in local part)
- Add CountByRegistrationIP to UserRepository interface
- Pass client IP to SendVerifyCodeAsync for abuse detection

Both checks return fake success to prevent enumeration attacks.

backend/internal/repository/user_repo.go

@@ -1113,3 +1113,26 @@ func (r *userRepository) DisableTotp(ctx context.Context, userID int64) error {
 	}
 	return nil
 }
+
+// CountByRegistrationIP 统计指定 IP 注册的用户数量
+func (r *userRepository) CountByRegistrationIP(ctx context.Context, ip string) (int, error) {
+	if strings.TrimSpace(ip) == "" {
+		return 0, nil
+	}
+	rows, err := r.sql.QueryContext(ctx,
+		`SELECT COUNT(*) FROM users WHERE register_ip_address = $1 AND deleted_at IS NULL`,
+		ip,
+	)
+	if err != nil {
+		return 0, err
+	}
+	defer rows.Close()
+
+	var count int
+	if rows.Next() {
+		if err := rows.Scan(&count); err != nil {
+			return 0, err
+		}
+	}
+	return count, nil
+}