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/DTOs/ServerDtos.cs

@@ -1,5 +1,10 @@
 namespace EchoHub.Core.DTOs;
 
-public record ServerStatusDto(string Name, string? Description, int OnlineUsers, int TotalChannels);
+public record ServerStatusDto(
+    string Name,
+    string? Description,
+    int OnlineUsers,
+    int TotalChannels,
+    string RegistrationMode = "open");
 
 public record EncryptionKeyResponse(string Key);