Add username validation and initial friends feature

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.

Govor.API.Tests/UnitTests/Services/Authentication/AuthServiceTests.cs

@@ -19,6 +19,7 @@ public class AuthServiceTests
     private Mock<IJwtService> _jwtServiceMock;
     private Mock<IUsersRepository> _usersRepositoryMock;
     private Mock<IAdminsRepository> _adminsRepositoryMock;
+    private Mock<IUsernameValidator> _usernameValidatorMock;
     
     private IAccountService _accountService;
     
@@ -38,12 +39,14 @@ public class AuthServiceTests
         _passwordHasherMock = new Mock<IPasswordHasher>();
         _jwtServiceMock = new Mock<IJwtService>();
         _adminsRepositoryMock = new Mock<IAdminsRepository>();
+        _usernameValidatorMock = new Mock<IUsernameValidator>();
         
         _accountService = new AuthService(
             _usersRepositoryMock.Object,
             _jwtServiceMock.Object,
             _passwordHasherMock.Object,
-            _adminsRepositoryMock.Object
+            _adminsRepositoryMock.Object,
+            _usernameValidatorMock.Object
             );
     }