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.
Introduced a UsernameValidator with Cyrillic and length checks, integrated into AuthService and registration flow. Added InvalidUsernameException and related interface. Updated User model to support friend requests, added FriendsController (stub), and configured Friendship entity in EF Core. Adjusted UserValidator max name length and removed navigation properties from PrivateChat.
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