feat: add IsPublic property to channels and enhance channel creation with visibility options

src/EchoHub.Core/Models/Channel.cs

@@ -5,6 +5,7 @@ public class Channel
     public Guid Id { get; set; }
     public required string Name { get; set; }
     public string? Topic { get; set; }
+    public bool IsPublic { get; set; } = true;
     public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
     public Guid CreatedByUserId { get; set; }