Implement username change functionality and enhance Telegram bot registration flow

- Added a new endpoint for changing usernames, allowing users to update their login credentials via the API.
- Integrated username change functionality into the settings page, providing a user-friendly interface for this action.
- Enhanced the Telegram bot to support user registration directly through the bot, including username generation and password delivery.
- Updated documentation to reflect the new username change endpoint and registration flow through the Telegram bot.

backend/src/PnvPanel.Application/Telegram/RegisterViaTelegramCommand.cs

@@ -0,0 +1,10 @@
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Telegram;
+
+/// <summary>Вызывается только из TelegramBotHostedService (обработка кнопки "Зарегистрироваться").</summary>
+public sealed record RegisterViaTelegramCommand(long TelegramUserId, string? TelegramUsername)
+    : ICommand<Result<RegisterViaTelegramResult>>;
+
+public sealed record RegisterViaTelegramResult(Guid UserId, string UserName, string Password);

backend/src/PnvPanel.Application/Telegram/RegisterViaTelegramCommandHandler.cs

@@ -0,0 +1,67 @@
+using System.Security.Cryptography;
+using PnvPanel.Application.Auth;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Telegram;
+
+public sealed class RegisterViaTelegramCommandHandler(IIdentityService identityService)
+    : ICommandHandler<RegisterViaTelegramCommand, Result<RegisterViaTelegramResult>>
+{
+    public async Task<Result<RegisterViaTelegramResult>> Handle(
+        RegisterViaTelegramCommand command, CancellationToken cancellationToken)
+    {
+        var existingUserId = await identityService.FindUserIdByTelegramUserIdAsync(command.TelegramUserId, cancellationToken);
+        if (existingUserId is not null)
+            return Result.Failure<RegisterViaTelegramResult>(TelegramErrors.AlreadyLinked);
+
+        var password = GeneratePassword();
+
+        // Сначала пробуем @username из Telegram как логин; если его нет или он уже занят на сайте —
+        // используем Telegram id (гарантированно уникален). Логин потом можно сменить в настройках.
+        var userName = command.TelegramUsername;
+        var createResult = string.IsNullOrWhiteSpace(userName)
+            ? Result.Failure<Guid>(AuthErrors.DuplicateUserName)
+            : await identityService.CreateUserAsync(userName, password, cancellationToken);
+
+        if (!createResult.IsSuccess)
+        {
+            userName = command.TelegramUserId.ToString();
+            createResult = await identityService.CreateUserAsync(userName, password, cancellationToken);
+            if (!createResult.IsSuccess)
+                return Result.Failure<RegisterViaTelegramResult>(createResult.Error);
+        }
+
+        await identityService.LinkTelegramAsync(createResult.Value, command.TelegramUserId, command.TelegramUsername, cancellationToken);
+
+        return Result.Success(new RegisterViaTelegramResult(createResult.Value, userName!, password));
+    }
+
+    /// <summary>
+    /// Гарантирует заглавную букву, строчную и цифру (см. Password.Require* в DependencyInjection.cs),
+    /// без спецсимволов (RequireNonAlphanumeric отключён) — присылается пользователю текстом в Telegram.
+    /// </summary>
+    private static string GeneratePassword()
+    {
+        const string upper = "ABCDEFGHJKLMNPQRSTUVWXYZ"; // без O/I — легко спутать при ручном вводе
+        const string lower = "abcdefghijkmnpqrstuvwxyz";
+        const string digits = "23456789";
+        const string all = upper + lower + digits;
+
+        Span<char> buffer = stackalloc char[12];
+        buffer[0] = upper[RandomNumberGenerator.GetInt32(upper.Length)];
+        buffer[1] = lower[RandomNumberGenerator.GetInt32(lower.Length)];
+        buffer[2] = digits[RandomNumberGenerator.GetInt32(digits.Length)];
+        for (var i = 3; i < buffer.Length; i++)
+            buffer[i] = all[RandomNumberGenerator.GetInt32(all.Length)];
+
+        for (var i = buffer.Length - 1; i > 0; i--)
+        {
+            var j = RandomNumberGenerator.GetInt32(i + 1);
+            (buffer[i], buffer[j]) = (buffer[j], buffer[i]);
+        }
+
+        return new string(buffer);
+    }
+}

backend/src/PnvPanel.Application/Telegram/TelegramErrors.cs

@@ -12,4 +12,7 @@ public static class TelegramErrors
 
     public static readonly Error NotLinked =
         Error.Conflict("Telegram.NotLinked", "Telegram не привязан ни к одному аккаунту.");
+
+    public static readonly Error AlreadyLinked =
+        Error.Conflict("Telegram.AlreadyLinked", "Этот Telegram уже привязан к аккаунту — используйте вход через Telegram.");
 }