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.Tests/Irc/IrcCommandHandlerTests.cs

@@ -340,8 +340,7 @@ public class IrcCommandHandlerTests
         var encryptedContent = _encryption.Encrypt("Hello from history!");
         _chatService.HistoryToReturn =
         [
-            new(Guid.NewGuid(), encryptedContent, "bob", null, "general",
-                MessageType.Text, null, null, DateTimeOffset.UtcNow)
+            new(Guid.NewGuid(), encryptedContent, "bob", null, "general", DateTimeOffset.UtcNow)
         ];
 
         var lines = await RunAuthenticated(["JOIN #general"]);