fix: add Discord OAuth token exchange logging for production diagnostics

- Log status code and response body when Discord /oauth2/token fails
- Helps identify why ExchangeCodeAsync returns null in production

Bump version → 2.8.1

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/GmRelay.Bot.Tests/Web/DiscordAuthServiceTests.cs

@@ -3,6 +3,7 @@ using System.Text.Json;
 using GmRelay.Web;
 using GmRelay.Web.Services;
 using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging.Abstractions;
 
 namespace GmRelay.Bot.Tests.Web;
 
@@ -20,7 +21,7 @@ public class DiscordAuthServiceTests
             })
             .Build();
 
-        var service = new DiscordAuthService(new TestHttpClientFactory(), config);
+        var service = new DiscordAuthService(new TestHttpClientFactory(), config, NullLogger<DiscordAuthService>.Instance);
         var url = service.BuildAuthorizeUrl("state123");
 
         Assert.Contains("client_id=12345", url);
@@ -33,7 +34,7 @@ public class DiscordAuthServiceTests
     public void BuildAuthorizeUrl_WithMissingConfig_ThrowsInvalidOperationException()
     {
         var config = new ConfigurationBuilder().Build();
-        var service = new DiscordAuthService(new TestHttpClientFactory(), config);
+        var service = new DiscordAuthService(new TestHttpClientFactory(), config, NullLogger<DiscordAuthService>.Instance);
 
         Assert.Throws<InvalidOperationException>(() => service.BuildAuthorizeUrl("state"));
     }
@@ -74,7 +75,7 @@ public class DiscordAuthServiceTests
             .Build();
 
         var factory = new TestHttpClientFactory(handler);
-        var service = new DiscordAuthService(factory, config);
+        var service = new DiscordAuthService(factory, config, NullLogger<DiscordAuthService>.Instance);
 
         var result = await service.ExchangeCodeAsync("valid_code");
 
@@ -102,7 +103,7 @@ public class DiscordAuthServiceTests
             .Build();
 
         var factory = new TestHttpClientFactory(handler);
-        var service = new DiscordAuthService(factory, config);
+        var service = new DiscordAuthService(factory, config, NullLogger<DiscordAuthService>.Instance);
 
         var result = await service.ExchangeCodeAsync("invalid_code");