Add Line webhook signature verification helper

Implements LineSignature.IsValid() using HMAC-SHA256 + FixedTimeEquals to prevent timing attacks; includes xUnit tests for valid, tampered, and null/empty header cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API.Tests/Services/Notifications/LineSignatureTests.cs

@@ -0,0 +1,47 @@
+using System.Security.Cryptography;
+using System.Text;
+using ROLAC.API.Services.Notifications;
+using Xunit;
+
+namespace ROLAC.API.Tests.Services.Notifications;
+
+public class LineSignatureTests
+{
+    private const string Secret = "test-channel-secret";
+
+    private static string Sign(string body)
+    {
+        using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(Secret));
+        return Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(body)));
+    }
+
+    [Fact]
+    public void IsValid_ReturnsTrue_ForMatchingSignature()
+    {
+        var body = """{"events":[]}""";
+        var signature = Sign(body);
+
+        var result = LineSignature.IsValid(Secret, Encoding.UTF8.GetBytes(body), signature);
+
+        Assert.True(result);
+    }
+
+    [Fact]
+    public void IsValid_ReturnsFalse_ForTamperedBody()
+    {
+        var signature = Sign("""{"events":[]}""");
+
+        var result = LineSignature.IsValid(Secret, Encoding.UTF8.GetBytes("""{"events":[1]}"""), signature);
+
+        Assert.False(result);
+    }
+
+    [Fact]
+    public void IsValid_ReturnsFalse_ForNullOrEmptyHeader()
+    {
+        var body = Encoding.UTF8.GetBytes("""{"events":[]}""");
+
+        Assert.False(LineSignature.IsValid(Secret, body, null));
+        Assert.False(LineSignature.IsValid(Secret, body, ""));
+    }
+}

API/ROLAC.API/Services/Notifications/LineSignature.cs

@@ -0,0 +1,20 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace ROLAC.API.Services.Notifications;
+
+/// <summary>Verifies the X-Line-Signature header (HMAC-SHA256 of the raw body, base64).</summary>
+public static class LineSignature
+{
+    public static bool IsValid(string channelSecret, byte[] rawBody, string? signatureHeader)
+    {
+        if (string.IsNullOrEmpty(signatureHeader)) return false;
+
+        using var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(channelSecret));
+        var expected = Convert.ToBase64String(hmac.ComputeHash(rawBody));
+
+        return CryptographicOperations.FixedTimeEquals(
+            Encoding.UTF8.GetBytes(expected),
+            Encoding.UTF8.GetBytes(signatureHeader));
+    }
+}