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.Application/Validators/UsernameValidator.cs

@@ -0,0 +1,35 @@
+using System.Linq.Expressions;
+using System.Text.RegularExpressions;
+using Govor.Application.Exceptions.AuthService;
+using Govor.Application.Interfaces.Authentication;
+using Govor.Core.Infrastructure.Validators;
+
+namespace Govor.Application.Validators;
+
+public class UsernameValidator : IUsernameValidator
+{
+    private readonly Regex _usernameRegex = new(@"^[А-Яа-яЁё]+$", RegexOptions.Compiled);
+    
+    public void Validate(string username)
+    {
+        if(username.Length < UserValidator.MIN_LENGHT_OF_NAME || username.Length > UserValidator.MAX_LENGHT_OF_NAME)
+            throw new InvalidUsernameException($"Username must be between {UserValidator.MIN_LENGHT_OF_NAME} and {UserValidator.MAX_LENGHT_OF_NAME} characters.");
+
+        if (!_usernameRegex.IsMatch(username))
+            throw new InvalidUsernameException("The username must be in Cyrillic and start with a letter.");
+    }
+
+    public bool TryValidate(string username)
+    {
+        try
+        {
+            Validate(username);
+            return true;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+}