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>

src/GmRelay.Web/Services/DiscordAuthService.cs

@@ -5,7 +5,7 @@ using System.Text.Json.Serialization;
 
 namespace GmRelay.Web.Services;
 
-public sealed class DiscordAuthService(IHttpClientFactory httpClientFactory, IConfiguration configuration)
+public sealed class DiscordAuthService(IHttpClientFactory httpClientFactory, IConfiguration configuration, ILogger<DiscordAuthService> logger)
 {
     private readonly DiscordOAuthOptions _options = configuration.GetSection("Discord").Get<DiscordOAuthOptions>() ?? new DiscordOAuthOptions();
 
@@ -40,10 +40,14 @@ public sealed class DiscordAuthService(IHttpClientFactory httpClientFactory, ICo
         });
 
         var response = await client.PostAsync("https://discord.com/api/oauth2/token", content);
-        if (!response.IsSuccessStatusCode)
-            return null;
-
         var json = await response.Content.ReadAsStringAsync();
+        if (!response.IsSuccessStatusCode)
+        {
+            logger.LogError("Discord token exchange failed: {StatusCode} {Body}. client_id={ClientId}, redirect_uri={RedirectUri}",
+                (int)response.StatusCode, json, _options.ClientId, _options.RedirectUri);
+            return null;
+        }
+
         return JsonSerializer.Deserialize<DiscordTokenResponse>(json);
     }