fix(discord): add console logging and deferred responses

- Add builder.Logging.AddConsole() to DiscordBot Program.cs so logs
  are visible in docker logs.
- Add granular LogInformation/LogError calls to DiscordNewSessionCommand
  and DiscordRescheduleCommand to diagnose failures.
- Use InteractionCallback.DeferredMessage() + ModifyResponseAsync pattern
  for /newsession and /reschedule to avoid Discord 3-second interaction
  timeout.
- Bump version → 3.0.8

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

src/GmRelay.DiscordBot/Program.cs

@@ -36,6 +36,8 @@ discordOptions.Validate();
 
 builder.Services.AddSingleton(discordOptions);
 
+builder.Logging.AddConsole();
+
 builder.Services.AddSingleton<NpgsqlDataSource>(sp =>
 {
     var config = sp.GetRequiredService<IConfiguration>();