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.Data/Configurations/MessagesConfiguration.cs

@@ -25,11 +25,6 @@ public class MessagesConfiguration : IEntityTypeConfiguration<Message>
             .HasForeignKey(mv => mv.MessageId)
             .OnDelete(DeleteBehavior.Cascade);
 
-        builder.HasOne(m => m.ReplyToMessage)
-            .WithMany()
-            .HasForeignKey(m => m.ReplyToMessageId)
-            .OnDelete(DeleteBehavior.Restrict); 
-
         builder.Property(m => m.EncryptedContent)
             .IsRequired();
     }