feat: classify Hanvon duplicate-face errors as DUPLICATE_FACE

Parse face is double,id=... separately from upload failures.

DbToDeviceFaceErrors.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Text.RegularExpressions;
+
+namespace HanvonF710XAttendanceService
+{
+    internal sealed class DbToDeviceDuplicateFaceEntry
+    {
+        public DbToDeviceDuplicateFaceEntry(string employeeId, string existingDeviceId, string targetDevice)
+        {
+            EmployeeId = employeeId ?? "";
+            ExistingDeviceId = existingDeviceId ?? "";
+            TargetDevice = targetDevice ?? "";
+        }
+
+        public string EmployeeId { get; }
+        public string ExistingDeviceId { get; }
+        public string TargetDevice { get; }
+    }
+
+    internal static class DbToDeviceFaceErrors
+    {
+        private static readonly Regex DuplicateFaceRegex = new Regex(
+            @"face\s+is\s+double\s*,\s*id\s*=\s*(\d+)",
+            RegexOptions.IgnoreCase | RegexOptions.CultureInvariant);
+
+        public static bool TryParseDuplicateFace(string errorMessage, out string existingDeviceId)
+        {
+            existingDeviceId = null;
+            if (string.IsNullOrWhiteSpace(errorMessage))
+            {
+                return false;
+            }
+
+            Match match = DuplicateFaceRegex.Match(errorMessage.Trim());
+            if (!match.Success || match.Groups.Count < 2)
+            {
+                return false;
+            }
+
+            existingDeviceId = match.Groups[1].Value?.Trim();
+            return !string.IsNullOrWhiteSpace(existingDeviceId);
+        }
+    }
+}

DbToDeviceFaceErrorsTests.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace HanvonF710XAttendanceService
+{
+    internal static class DbToDeviceFaceErrorsTests
+    {
+        public static int RunAll()
+        {
+            int failed = 0;
+            failed += Run("parse duplicate face error", TestParseDuplicateFace);
+            failed += Run("ignore generic upload error", TestIgnoreGenericError);
+            return failed;
+        }
+
+        private static int Run(string name, Func<bool> test)
+        {
+            try
+            {
+                if (!test())
+                {
+                    Console.WriteLine("FAIL: " + name);
+                    return 1;
+                }
+
+                Console.WriteLine("PASS: " + name);
+                return 0;
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine("FAIL: " + name + " err=" + ex.Message);
+                return 1;
+            }
+        }
+
+        private static bool TestParseDuplicateFace()
+        {
+            return DbToDeviceFaceErrors.TryParseDuplicateFace("face is double,id=6400", out string existingId)
+                && existingId == "6400"
+                && DbToDeviceFaceErrors.TryParseDuplicateFace("Face is double, id=6475", out string existingId2)
+                && existingId2 == "6475";
+        }
+
+        private static bool TestIgnoreGenericError()
+        {
+            return !DbToDeviceFaceErrors.TryParseDuplicateFace("no face in picture", out _);
+        }
+    }
+}