feat: implement link embed functionality to enhance message previews with metadata and thumbnails

src/EchoHub.Core/DTOs/ChatDtos.cs

@@ -11,7 +11,8 @@ public record MessageDto(
     MessageType Type,
     string? AttachmentUrl,
     string? AttachmentFileName,
-    DateTimeOffset SentAt);
+    DateTimeOffset SentAt,
+    EmbedDto? Embed = null);
 
 public record ChannelDto(
     Guid Id,
@@ -36,3 +37,10 @@ public record CreateChannelRequest(string Name, string? Topic = null, bool IsPub
 public record UpdateTopicRequest(string? Topic);
 
 public record SendUrlRequest(string Url);
+
+public record EmbedDto(
+    string? SiteName,
+    string? Title,
+    string? Description,
+    string? ImageAscii,
+    string Url);