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/Models/AttachmentKind.cs

@@ -0,0 +1,12 @@
+namespace EchoHub.Core.Models;
+
+/// <summary>
+/// The kind of a message attachment. Determines how the client renders it
+/// (ASCII preview for images, a play affordance for audio, a download line for files).
+/// </summary>
+public enum AttachmentKind
+{
+    Image,
+    Audio,
+    File
+}