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/Models/InviteCode.cs

@@ -0,0 +1,20 @@
+namespace EchoHub.Core.Models;
+
+/// <summary>
+/// A registration invite code. When the server runs with <c>Server:Registration = "invite"</c>,
+/// new accounts (REST and IRC alike) require a valid, unexpired, not-fully-used code.
+/// </summary>
+public class InviteCode
+{
+    public Guid Id { get; set; }
+    public required string Code { get; set; }
+    public Guid CreatedByUserId { get; set; }
+    public string CreatedByUsername { get; set; } = string.Empty;
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+
+    /// <summary>Null means the code never expires.</summary>
+    public DateTimeOffset? ExpiresAt { get; set; }
+
+    public int MaxUses { get; set; } = 1;
+    public int UseCount { get; set; }
+}