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/Infrastructure/Logging/DiscordGatewayLifecycleLogger.cs

@@ -0,0 +1,43 @@
+using NetCord.Gateway;
+using NetCord.Hosting.Gateway;
+
+namespace GmRelay.DiscordBot.Infrastructure.Logging;
+
+public sealed class DiscordGatewayLifecycleLogger(
+    ILogger<DiscordGatewayLifecycleLogger> logger)
+    : IConnectGatewayHandler,
+      IReadyGatewayHandler,
+      IDisconnectGatewayHandler,
+      IResumeGatewayHandler
+{
+    public ValueTask HandleAsync()
+    {
+        logger.LogInformation("Discord gateway connected");
+        return ValueTask.CompletedTask;
+    }
+
+    public ValueTask HandleAsync(ReadyEventArgs arg)
+    {
+        logger.LogInformation(
+            "Discord gateway ready for application {ApplicationId} in {GuildCount} guilds",
+            arg.ApplicationId,
+            arg.GuildIds.Count);
+
+        return ValueTask.CompletedTask;
+    }
+
+    public ValueTask HandleAsync(DisconnectEventArgs arg)
+    {
+        logger.LogWarning(
+            "Discord gateway disconnected; reconnect scheduled: {Reconnect}",
+            arg.Reconnect);
+
+        return ValueTask.CompletedTask;
+    }
+
+    ValueTask IResumeGatewayHandler.HandleAsync()
+    {
+        logger.LogInformation("Discord gateway session resumed");
+        return ValueTask.CompletedTask;
+    }
+}

src/GmRelay.DiscordBot/Infrastructure/Logging/SecretRedactor.cs

@@ -0,0 +1,14 @@
+using System.Text.RegularExpressions;
+
+namespace GmRelay.DiscordBot.Infrastructure.Logging;
+
+internal static partial class SecretRedactor
+{
+    public static string RedactConnectionString(string connectionString)
+    {
+        return PasswordPattern().Replace(connectionString, "$1***");
+    }
+
+    [GeneratedRegex(@"(?i)(Password\s*=\s*)[^;]+")]
+    private static partial Regex PasswordPattern();
+}