Refactor media attachments to use MediaFile entity

This commit introduces the MediaFile entity and updates the media attachments model, repositories, and related logic to reference MediaFile instead of storing file metadata directly in MediaAttachments. Controller, service, and SignalR request/response contracts are updated to support the new structure. Database configurations and validators are also adjusted accordingly. This refactor improves media management and normalization in the data model.

Govor.Contracts/Requests/SignalR/EditMessageRequest.cs

@@ -0,0 +1,7 @@
+namespace Govor.Contracts.Requests.SignalR;
+
+public class EditMessageRequest
+{
+    public Guid MessageId { get; set; }
+    public string NewEncryptedContent { get; set; } = string.Empty;
+}

Govor.Contracts/Requests/SignalR/MediaReference.cs

@@ -6,6 +6,4 @@ public record MediaReference
 {
     public Guid MediaId { get; init; }
     public string EncryptedKey { get; init; } = string.Empty;
-    public MediaType Type { get; init; }
-    public string MimeType { get; init; } = string.Empty;
 }

Govor.Contracts/Requests/SignalR/RemoveMessageRequest.cs

@@ -0,0 +1,6 @@
+namespace Govor.Contracts.Requests.SignalR;
+
+public class RemoveMessageRequest
+{
+    public Guid MessageId { get; set; }
+}