refactor: add platform messenger contracts

Introduce platform-neutral PlatformKind, PlatformUser, PlatformGroup, and IPlatformMessenger contracts in GmRelay.Shared.

Route Telegram session schedule updates, direct notifications, interaction replies, and calendar export through TelegramPlatformMessenger while preserving existing Telegram behavior.

Bump version -> 2.0.1

src/GmRelay.Shared/Platform/IPlatformMessenger.cs

@@ -0,0 +1,16 @@
+namespace GmRelay.Shared.Platform;
+
+public interface IPlatformMessenger
+{
+    Task<PlatformMessageRef> SendScheduleAsync(PlatformScheduleMessage message, CancellationToken ct);
+
+    Task UpdateScheduleAsync(PlatformScheduleMessage message, CancellationToken ct);
+
+    Task SendGroupMessageAsync(PlatformGroup group, string htmlText, CancellationToken ct);
+
+    Task SendPrivateMessageAsync(PlatformPrivateMessage message, CancellationToken ct);
+
+    Task AnswerInteractionAsync(PlatformInteractionReply reply, CancellationToken ct);
+
+    Task SendCalendarFileAsync(PlatformCalendarFile file, CancellationToken ct);
+}

src/GmRelay.Shared/Platform/PlatformGroup.cs

@@ -0,0 +1,8 @@
+namespace GmRelay.Shared.Platform;
+
+public sealed record PlatformGroup(
+    PlatformKind Platform,
+    string ExternalGroupId,
+    string DisplayName,
+    string? ExternalChannelId = null,
+    string? ExternalThreadId = null);

src/GmRelay.Shared/Platform/PlatformKind.cs

@@ -0,0 +1,8 @@
+namespace GmRelay.Shared.Platform;
+
+public enum PlatformKind
+{
+    Telegram = 0,
+    Discord = 1,
+    Max = 2
+}

src/GmRelay.Shared/Platform/PlatformMessageContracts.cs

@@ -0,0 +1,36 @@
+using GmRelay.Shared.Rendering;
+
+namespace GmRelay.Shared.Platform;
+
+public sealed record PlatformMessageRef(
+    PlatformKind Platform,
+    string ExternalGroupId,
+    string? ExternalThreadId,
+    string ExternalMessageId);
+
+public sealed record PlatformMessageAction(
+    string Key,
+    string Label,
+    string Payload);
+
+public sealed record PlatformScheduleMessage(
+    PlatformGroup Group,
+    SessionBatchViewModel View,
+    PlatformMessageRef? ExistingMessage,
+    string? ImageReference = null);
+
+public sealed record PlatformPrivateMessage(
+    PlatformUser Recipient,
+    string HtmlText);
+
+public sealed record PlatformInteractionReply(
+    string InteractionId,
+    string Text,
+    bool ShowAlert = false);
+
+public sealed record PlatformCalendarFile(
+    PlatformGroup Group,
+    string FileName,
+    byte[] Content,
+    string CaptionHtml,
+    IReadOnlyList<PlatformMessageAction> Actions);

src/GmRelay.Shared/Platform/PlatformUser.cs

@@ -0,0 +1,7 @@
+namespace GmRelay.Shared.Platform;
+
+public sealed record PlatformUser(
+    PlatformKind Platform,
+    string ExternalUserId,
+    string DisplayName,
+    string? ExternalUsername);