feat: Add support for message attachments

- Introduced Attachment model to handle file attachments associated with messages.
- Updated ModerationController to manage message deletions and attachment cleanup.
- Enhanced ChatService to include attachments in message retrieval.
- Implemented migration for legacy single-attachment messages to the new Attachments model.
- Added unit tests for attachment handling in message formatting and parsing.
- Updated database context and migrations to support new Attachments table.

src/EchoHub.Core/DTOs/ChatDtos.cs

@@ -8,13 +8,22 @@ public record MessageDto(
     string SenderUsername,
     string? SenderNicknameColor,
     string ChannelName,
-    MessageType Type,
-    string? AttachmentUrl,
-    string? AttachmentFileName,
     DateTimeOffset SentAt,
-    long? AttachmentFileSize = null,
+    List<AttachmentDto>? Attachments = null,
     List<EmbedDto>? Embeds = null);
 
+/// <summary>
+/// A file attached to a message. <see cref="AsciiPreview"/> holds the color-tag art for
+/// images (null otherwise). For end-to-end encrypted channels the content behind
+/// <see cref="Url"/> and the preview are ciphertext the server cannot read.
+/// </summary>
+public record AttachmentDto(
+    AttachmentKind Kind,
+    string Url,
+    string FileName,
+    long FileSize,
+    string? AsciiPreview = null);
+
 public record ChannelDto(
     Guid Id,
     string Name,