feat: integrate Serilog and Telegram notifications

- Add Serilog for structured logging (Console + File)
- Implement TelegramNotificationService for admin alerts
- Log successful/failed login attempts with Telegram notifications
- Add application startup/shutdown logging
- Log important events to Telegram Chat ID: -5585148480
- Configuration: Telegram:BotToken and Telegram:ChatId in appsettings

Features:
- Automatic daily log rotation
- Structured logging with timestamps
- Environment-aware alerts
- Error and info level Telegram messages

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

TaxBaik.Web/Services/AuthService.cs

@@ -12,15 +12,21 @@ public class AuthService
 {
     private readonly IAdminUserRepository _adminUserRepository;
     private readonly ILogger<AuthService> _logger;
+    private readonly ITelegramNotificationService _telegramService;
     private readonly string _jwtSecretKey;
     private readonly string? _passwordResetToken;
     private readonly int _accessTokenExpirationMinutes = 60;    // Access Token: 1시간 (사용성 향상)
     private readonly int _refreshTokenExpirationMinutes = 10080; // Refresh Token: 7일
 
-    public AuthService(IAdminUserRepository adminUserRepository, ILogger<AuthService> logger, IConfiguration configuration)
+    public AuthService(
+        IAdminUserRepository adminUserRepository,
+        ILogger<AuthService> logger,
+        IConfiguration configuration,
+        ITelegramNotificationService telegramService)
     {
         _adminUserRepository = adminUserRepository;
         _logger = logger;
+        _telegramService = telegramService;
         _jwtSecretKey = configuration["Jwt:SecretKey"] ?? throw new InvalidOperationException("Missing 'Jwt:SecretKey' configuration.");
         _passwordResetToken = configuration["Admin:PasswordResetToken"];
     }
@@ -46,10 +52,16 @@ public class AuthService
         if (!BCrypt.Net.BCrypt.Verify(password, user.PasswordHash))
         {
             _logger.LogWarning("로그인 시도: 잘못된 비밀번호 '{Username}'", username);
+            await _telegramService.SendErrorAsync(
+                "로그인 실패",
+                $"사용자: {username}\n실패 사유: 잘못된 비밀번호");
             return null;
         }
 
         _logger.LogInformation("로그인 성공: {Username}", username);
+        await _telegramService.SendInfoAsync(
+            "관리자 로그인",
+            $"사용자: {username}");
         await _adminUserRepository.UpdateLastLoginAtAsync(user.Id);
         return GenerateTokenPair(user);
     }