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.DiscordBot/Infrastructure/Discord/DiscordInteractionReplyCache.cs

@@ -0,0 +1,17 @@
+using System.Collections.Concurrent;
+using GmRelay.Shared.Platform;
+
+namespace GmRelay.DiscordBot.Infrastructure.Discord;
+
+public sealed class DiscordInteractionReplyCache
+{
+    private readonly ConcurrentDictionary<string, PlatformInteractionReply> replies = new(StringComparer.Ordinal);
+
+    public void Store(PlatformInteractionReply reply) =>
+        replies[reply.InteractionId] = reply;
+
+    public PlatformInteractionReply? Take(string interactionId) =>
+        replies.TryRemove(interactionId, out var reply)
+            ? reply
+            : null;
+}

src/GmRelay.DiscordBot/Infrastructure/Discord/DiscordPlatformMessenger.cs

@@ -6,7 +6,9 @@ using NetCord.Rest;
 
 namespace GmRelay.DiscordBot.Infrastructure.Discord;
 
-public sealed class DiscordPlatformMessenger(RestClient restClient) : IPlatformMessenger
+public sealed class DiscordPlatformMessenger(
+    RestClient restClient,
+    DiscordInteractionReplyCache interactionReplies) : IPlatformMessenger
 {
     public async Task<PlatformMessageRef> SendScheduleAsync(PlatformScheduleMessage message, CancellationToken ct)
     {
@@ -61,6 +63,7 @@ public sealed class DiscordPlatformMessenger(RestClient restClient) : IPlatformM
 
     public Task AnswerInteractionAsync(PlatformInteractionReply reply, CancellationToken ct)
     {
+        interactionReplies.Store(reply);
         return Task.CompletedTask;
     }