feat: add Discord NetCord gateway worker

Add a separate GmRelay.DiscordBot worker using NetCord Gateway with startup token validation, PostgreSQL datasource registration, slash-command setup, component interaction service registration, and lifecycle logging.

Wire the Discord service through Aspire AppHost, Docker Compose, PR checks, deploy image build/push/scan/pull steps, README docs, and synchronized version 2.2.0.

Add TDD coverage for project isolation, token validation, startup wiring, runtime wiring, and version synchronization.

Bump version -> 2.2.0

src/GmRelay.DiscordBot/Program.cs

@@ -0,0 +1,54 @@
+using GmRelay.DiscordBot;
+using GmRelay.DiscordBot.Infrastructure.Logging;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using NetCord;
+using NetCord.Gateway;
+using NetCord.Hosting.Gateway;
+using NetCord.Hosting.Services.ApplicationCommands;
+using NetCord.Hosting.Services.ComponentInteractions;
+using NetCord.Services.ComponentInteractions;
+using Npgsql;
+
+var builder = Host.CreateApplicationBuilder(args);
+
+builder.AddServiceDefaults();
+
+var discordOptions = builder.Configuration
+    .GetRequiredSection("Discord")
+    .Get<DiscordOptions>() ?? new DiscordOptions();
+discordOptions.Validate();
+
+builder.Services.AddSingleton(discordOptions);
+
+builder.Services.AddSingleton<NpgsqlDataSource>(sp =>
+{
+    var config = sp.GetRequiredService<IConfiguration>();
+    var loggerFactory = sp.GetRequiredService<ILoggerFactory>();
+    var connectionString = config.GetConnectionString("gmrelaydb")
+        ?? throw new InvalidOperationException(
+            "ConnectionStrings:gmrelaydb is required. Set via environment variable ConnectionStrings__gmrelaydb.");
+
+    var logger = loggerFactory.CreateLogger("GmRelay.DiscordBot.Startup");
+    logger.LogInformation(
+        "Configured PostgreSQL data source with connection string {ConnectionString}",
+        SecretRedactor.RedactConnectionString(connectionString));
+
+    return NpgsqlDataSource.Create(connectionString);
+});
+
+builder.Services
+    .AddDiscordGateway(options =>
+    {
+        options.Token = discordOptions.Token;
+        options.Intents = GatewayIntents.Guilds;
+    })
+    .AddApplicationCommands()
+    .AddComponentInteractions<ButtonInteraction, ButtonInteractionContext>()
+    .AddGatewayHandlers(typeof(Program).Assembly);
+
+var host = builder.Build();
+
+host.AddSlashCommand("ping", "Checks whether GM-Relay Discord is online.", () => "Pong!");
+
+await host.RunAsync();