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

src/EchoHub.Core/Constants/HubConstants.cs

@@ -13,4 +13,11 @@ public static class HubConstants
     public const int AsciiArtWidth = 80;
     public const int AsciiArtHeight = 40;
     public const int AsciiArtHeightHalfBlock = 80;
+
+    // Link embed constants
+    public const int EmbedThumbnailWidth = 24;
+    public const int EmbedThumbnailHeight = 12;
+    public const int EmbedMaxDescriptionLength = 200;
+    public const int EmbedMaxHtmlBytes = 64 * 1024; // 64 KB
+    public const int EmbedFetchTimeoutSeconds = 3;
 }

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);

src/EchoHub.Core/Models/Message.cs

@@ -7,6 +7,7 @@ public class Message
     public MessageType Type { get; set; } = MessageType.Text;
     public string? AttachmentUrl { get; set; }
     public string? AttachmentFileName { get; set; }
+    public string? EmbedJson { get; set; }
     public DateTimeOffset SentAt { get; set; } = DateTimeOffset.UtcNow;
 
     public Guid ChannelId { get; set; }