feat(discord): enable session join leave buttons

Move neutral join/leave handlers into GmRelay.Shared so Telegram and Discord share capacity, waitlist, duplicate-click, and schedule-update behavior.

Add Discord component routing for join_session and leave_session buttons with deferred ephemeral replies and serialized schedule message updates.

Bump version to 2.5.0 and update Discord docs.

Refs #29

src/GmRelay.Shared/Features/Sessions/CreateSession/ScheduleMessageUpdateLock.cs

@@ -0,0 +1,39 @@
+using System.Collections.Concurrent;
+using GmRelay.Shared.Platform;
+
+namespace GmRelay.Shared.Features.Sessions.CreateSession;
+
+public interface IScheduleMessageUpdateLock
+{
+    ValueTask<IAsyncDisposable> AcquireAsync(PlatformMessageRef scheduleMessage, CancellationToken ct);
+}
+
+public sealed class ScheduleMessageUpdateLock : IScheduleMessageUpdateLock
+{
+    private readonly ConcurrentDictionary<string, SemaphoreSlim> locks = new(StringComparer.Ordinal);
+
+    public async ValueTask<IAsyncDisposable> AcquireAsync(PlatformMessageRef scheduleMessage, CancellationToken ct)
+    {
+        var key = CreateKey(scheduleMessage);
+        var semaphore = locks.GetOrAdd(key, _ => new SemaphoreSlim(1, 1));
+        await semaphore.WaitAsync(ct);
+        return new Releaser(semaphore);
+    }
+
+    private static string CreateKey(PlatformMessageRef scheduleMessage) =>
+        string.Join(
+            '\u001F',
+            scheduleMessage.Platform.ToString(),
+            scheduleMessage.ExternalGroupId,
+            scheduleMessage.ExternalThreadId ?? string.Empty,
+            scheduleMessage.ExternalMessageId);
+
+    private sealed class Releaser(SemaphoreSlim semaphore) : IAsyncDisposable
+    {
+        public ValueTask DisposeAsync()
+        {
+            semaphore.Release();
+            return ValueTask.CompletedTask;
+        }
+    }
+}