Split IFriendRequestService into command and query interfaces, refactor related services and tests, and update dependency injection. Add HubResult response model and a SignalR HubExceptionFilter for consistent error handling. Move and update FriendsHub to use new command service and result pattern. Update username validation to allow digits after Cyrillic letters. Add new controllers and configuration for SignalR. Remove obsolete IFriendRequestService and related code.
Introduced ICurrentUserService and its implementation to centralize retrieval of the current user's ID from claims. Updated FriendsController to use this service instead of direct claim access. Registered the service and IHttpContextAccessor in DI. Added integration tests for FriendsController. Moved UsernameValidator to Infrastructure/Validators.
Artemy