feat: Add invite codes and message replies functionality

- Introduced a new migration to add InviteCodes table and ReplyToMessageId column in Messages.
- Updated ChatHub to support replying to messages.
- Enhanced ChatService to handle message replies and validate reply targets.
- Modified UserService to implement invite-only registration mode with invite code consumption.
- Added configuration options for registration modes in appsettings.
- Created unit tests for new features including invite code registration and message reply formatting.

src/EchoHub.Core/Contracts/IChatService.cs

@@ -15,8 +15,9 @@ public interface IChatService
 
     // Messaging. originConnectionId identifies the connection the message came from so
     // broadcasters can avoid echoing it back to that one connection (IRC convention);
-    // the sender's other sessions still receive it.
-    Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content, string? originConnectionId = null);
+    // the sender's other sessions still receive it. replyToMessageId references the message
+    // being replied to; it must exist in the same channel.
+    Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content, string? originConnectionId = null, Guid? replyToMessageId = null);
     Task<List<MessageDto>> GetChannelHistoryAsync(string channelName, int count, int offset = 0);
 
     // Presence

src/EchoHub.Core/Contracts/IUserService.cs

@@ -4,7 +4,9 @@ namespace EchoHub.Core.Contracts;
 
 public interface IUserService
 {
-    Task<UserOperationResult> RegisterUserAsync(string username, string password, string? displayName = null);
+    // inviteCode is required when the server runs with Server:Registration = "invite";
+    // "closed" refuses all new accounts. Both REST and the IRC gateway funnel through here.
+    Task<UserOperationResult> RegisterUserAsync(string username, string password, string? displayName = null, string? inviteCode = null);
     Task<UserOperationResult> AuthenticateUserAsync(string username, string password);
     Task<UserProfileDto?> GetUserProfileAsync(string username);
     Task<UserProfileDto?> GetUserByIdAsync(Guid userId);