feat: detect active registered machine users for skip logic

Active serial lookup used before device enrollment.

AttendanceMachineUserDAO.cs

@@ -122,6 +122,61 @@ namespace HanvonF710XAttendanceService
             }
         }
 
+        /// <summary>
+        /// True when attendance_machine_user has an active row for this machine + device serial.
+        /// </summary>
+        public bool IsActiveRegistered(MySqlConnection connection, string machineId, string serialNumber)
+        {
+            if (string.IsNullOrWhiteSpace(machineId) || string.IsNullOrWhiteSpace(serialNumber))
+            {
+                return false;
+            }
+
+            const string query =
+                "SELECT 1 FROM hrms.attendance_machine_user " +
+                "WHERE machine_id = @MachineId AND serial_number = @SerialNumber AND is_deleted = 0 LIMIT 1";
+            using (var cmd = new MySqlCommand(query, connection))
+            {
+                cmd.CommandTimeout = 60;
+                cmd.Parameters.AddWithValue("@MachineId", machineId);
+                cmd.Parameters.AddWithValue("@SerialNumber", serialNumber.Trim());
+                var result = cmd.ExecuteScalar();
+                return result != null && result != DBNull.Value;
+            }
+        }
+
+        public HashSet<string> GetActiveSerialNumbersForMachine(MySqlConnection connection, string machineId)
+        {
+            var serials = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+            if (string.IsNullOrWhiteSpace(machineId))
+            {
+                return serials;
+            }
+
+            const string query =
+                "SELECT serial_number FROM hrms.attendance_machine_user " +
+                "WHERE machine_id = @MachineId AND is_deleted = 0";
+            using (var cmd = new MySqlCommand(query, connection))
+            {
+                cmd.CommandTimeout = 120;
+                cmd.Parameters.AddWithValue("@MachineId", machineId);
+                using (var reader = cmd.ExecuteReader())
+                {
+                    while (reader.Read())
+                    {
+                        if (reader.IsDBNull(0)) continue;
+                        var s = reader.GetString(0)?.Trim();
+                        if (!string.IsNullOrWhiteSpace(s))
+                        {
+                            serials.Add(s);
+                        }
+                    }
+                }
+            }
+
+            return serials;
+        }
+
         private List<AttendanceMachineUser> getDBUsers(MySqlDataReader reader)
         {
             List<AttendanceMachineUser> DbUsers = new List<AttendanceMachineUser>();