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/FriendsController.cs

@@ -0,0 +1,56 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Govor.API.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+[Authorize(Roles = "User,Admin")]
+public class FriendsController : Controller
+{
+    private readonly ILogger<FriendsController> _logger;
+
+    [HttpGet("search")]
+    public async Task<IActionResult> Search([FromBody] string query)
+    {
+        return BadRequest("Not a valid request");
+    }
+
+    [HttpPost("request")]
+    public async Task<IActionResult> SendRequest([FromBody] Guid targetUserId)
+    {
+        return BadRequest("Not a valid request");
+    }
+
+    [HttpGet("requests")]
+    public async Task<IActionResult> GetIncomingRequests()
+    {
+        return BadRequest("Not a valid request");
+    }
+
+    [HttpPost("accept")]
+    public async Task<IActionResult> AcceptFriend([FromBody] Guid requesterId)
+    {
+        return BadRequest("Not a valid request");
+    }
+
+    [HttpGet]
+    public async Task<IActionResult> GetFriends()
+    {
+        return BadRequest("Not a valid request");
+    }
+
+    private Guid GetCurrentUserId()
+    {
+        var userIdClaim = HttpContext.User?.FindFirst("userID")?.Value;
+        _logger.LogInformation("Claims: {Claims}", string.Join(", ", HttpContext.User?.Claims.Select(c => $"{c.Type}: {c.Value}") ?? new string[0]));
+        
+        if (string.IsNullOrEmpty(userIdClaim))
+        {
+            _logger.LogError("No userID claim found");
+            return Guid.Empty;
+        }
+        
+        return Guid.TryParse(userIdClaim, out var userId) ? userId : Guid.Empty;
+    }
+}