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/Controllers/AuthController.cs

@@ -35,7 +35,8 @@ public class AuthController : Controller
 
             var invite = _invitesService.Validate(registrationRequest.InviteLink);
 
-            var token = await _accountService.RegistrationAsync(registrationRequest.Name, registrationRequest.Password, invite);
+            var token = await _accountService.RegistrationAsync(registrationRequest.Name, registrationRequest.Password,
+                invite);
             _logger.LogInformation($"Register request for {registrationRequest.Name}");
             return Ok(new { token });
         }
@@ -49,6 +50,11 @@ public class AuthController : Controller
             _logger.LogWarning(ex, $"Invite link invalid: {registrationRequest.InviteLink}");
             return BadRequest("Invite link invalid.");
         }
+        catch (InvalidUsernameException ex)
+        {
+            _logger.LogWarning(ex, $"Invalid username: {registrationRequest.Name}");
+            return BadRequest($"Invalid username: {ex.Message}");
+        }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Unexpected error during registration for user {Name}", registrationRequest.Name);