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/AnnouncementBrowserClient.cs

@@ -1,5 +1,6 @@
 namespace TaxBaik.Web.Services;
 
+using System.Net.Http;
 using System.Net.Http.Json;
 using TaxBaik.Application.DTOs;
 using TaxBaik.Domain.Entities;
@@ -17,17 +18,28 @@ public class AnnouncementBrowserClient : IAnnouncementBrowserClient
 {
     private readonly HttpClient _http;
     private readonly ILogger<AnnouncementBrowserClient> _logger;
+    private readonly ITokenStore _tokenStore;
 
-    public AnnouncementBrowserClient(HttpClient http, ILogger<AnnouncementBrowserClient> logger)
+    public AnnouncementBrowserClient(HttpClient http, ILogger<AnnouncementBrowserClient> logger, ITokenStore tokenStore)
     {
         _http = http;
         _logger = logger;
+        _tokenStore = tokenStore;
+    }
+
+    private void EnsureAuthHeader()
+    {
+        if (!string.IsNullOrEmpty(_tokenStore.AccessToken) && !_http.DefaultRequestHeaders.Contains("Authorization"))
+        {
+            _http.DefaultRequestHeaders.Authorization = new("Bearer", _tokenStore.AccessToken);
+        }
     }
 
     public async Task<IEnumerable<Announcement>> GetAllAsync(CancellationToken ct = default)
     {
         try
         {
+            EnsureAuthHeader();
             var result = await _http.GetFromJsonAsync<AnnouncementListResponse>("announcement", cancellationToken: ct);
             return result?.Data ?? [];
         }
@@ -42,6 +54,7 @@ public class AnnouncementBrowserClient : IAnnouncementBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             return await _http.GetFromJsonAsync<Announcement>($"announcement/{id}", cancellationToken: ct);
         }
         catch (HttpRequestException ex)
@@ -55,6 +68,7 @@ public class AnnouncementBrowserClient : IAnnouncementBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PostAsJsonAsync("announcement", dto, cancellationToken: ct);
             if (!response.IsSuccessStatusCode) return null;
 
@@ -74,6 +88,7 @@ public class AnnouncementBrowserClient : IAnnouncementBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PutAsJsonAsync($"announcement/{id}", dto, cancellationToken: ct);
             if (!response.IsSuccessStatusCode) return null;
 
@@ -93,6 +108,7 @@ public class AnnouncementBrowserClient : IAnnouncementBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.DeleteAsync($"announcement/{id}", cancellationToken: ct);
             return response.IsSuccessStatusCode;
         }

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);
     }

TaxBaik.Web/Services/ClientBrowserClient.cs

@@ -1,5 +1,6 @@
 namespace TaxBaik.Web.Services;
 
+using System.Net.Http;
 using System.Net.Http.Json;
 using TaxBaik.Application.DTOs;
 using TaxBaik.Domain.Entities;
@@ -22,11 +23,21 @@ public class ClientBrowserClient : IClientBrowserClient
 {
     private readonly HttpClient _http;
     private readonly ILogger<ClientBrowserClient> _logger;
+    private readonly ITokenStore _tokenStore;
 
-    public ClientBrowserClient(HttpClient http, ILogger<ClientBrowserClient> logger)
+    public ClientBrowserClient(HttpClient http, ILogger<ClientBrowserClient> logger, ITokenStore tokenStore)
     {
         _http = http;
         _logger = logger;
+        _tokenStore = tokenStore;
+    }
+
+    private void EnsureAuthHeader()
+    {
+        if (!string.IsNullOrEmpty(_tokenStore.AccessToken) && !_http.DefaultRequestHeaders.Contains("Authorization"))
+        {
+            _http.DefaultRequestHeaders.Authorization = new("Bearer", _tokenStore.AccessToken);
+        }
     }
 
     public async Task<(IEnumerable<Client> Items, int Total)> GetPagedAsync(
@@ -34,6 +45,7 @@ public class ClientBrowserClient : IClientBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var query = $"client?page={page}&pageSize={pageSize}";
             if (!string.IsNullOrEmpty(status))
                 query += $"&status={status}";
@@ -54,6 +66,7 @@ public class ClientBrowserClient : IClientBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             return await _http.GetFromJsonAsync<Client>($"client/{id}", cancellationToken: ct);
         }
         catch (HttpRequestException ex)
@@ -67,6 +80,7 @@ public class ClientBrowserClient : IClientBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PostAsJsonAsync("client", dto, cancellationToken: ct);
             if (!response.IsSuccessStatusCode)
                 return null;
@@ -87,6 +101,7 @@ public class ClientBrowserClient : IClientBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PutAsJsonAsync($"client/{id}", dto, cancellationToken: ct);
             if (!response.IsSuccessStatusCode)
                 return null;
@@ -107,6 +122,7 @@ public class ClientBrowserClient : IClientBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.DeleteAsync($"client/{id}", cancellationToken: ct);
             return response.IsSuccessStatusCode;
         }

TaxBaik.Web/Services/FaqBrowserClient.cs

@@ -1,5 +1,6 @@
 namespace TaxBaik.Web.Services;
 
+using System.Net.Http;
 using System.Net.Http.Json;
 using TaxBaik.Domain.Entities;
 
@@ -16,17 +17,28 @@ public class FaqBrowserClient : IFaqBrowserClient
 {
     private readonly HttpClient _http;
     private readonly ILogger<FaqBrowserClient> _logger;
+    private readonly ITokenStore _tokenStore;
 
-    public FaqBrowserClient(HttpClient http, ILogger<FaqBrowserClient> logger)
+    public FaqBrowserClient(HttpClient http, ILogger<FaqBrowserClient> logger, ITokenStore tokenStore)
     {
         _http = http;
         _logger = logger;
+        _tokenStore = tokenStore;
+    }
+
+    private void EnsureAuthHeader()
+    {
+        if (!string.IsNullOrEmpty(_tokenStore.AccessToken) && !_http.DefaultRequestHeaders.Contains("Authorization"))
+        {
+            _http.DefaultRequestHeaders.Authorization = new("Bearer", _tokenStore.AccessToken);
+        }
     }
 
     public async Task<IEnumerable<Faq>> GetAllAsync(CancellationToken ct = default)
     {
         try
         {
+            EnsureAuthHeader();
             var result = await _http.GetFromJsonAsync<FaqListResponse>("faq", cancellationToken: ct);
             return result?.Data ?? [];
         }
@@ -41,6 +53,7 @@ public class FaqBrowserClient : IFaqBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             return await _http.GetFromJsonAsync<Faq>($"faq/{id}", cancellationToken: ct);
         }
         catch (HttpRequestException ex)
@@ -54,6 +67,7 @@ public class FaqBrowserClient : IFaqBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PostAsJsonAsync("faq", faq, cancellationToken: ct);
             if (!response.IsSuccessStatusCode) return null;
 
@@ -73,6 +87,7 @@ public class FaqBrowserClient : IFaqBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PutAsJsonAsync($"faq/{id}", faq, cancellationToken: ct);
             if (!response.IsSuccessStatusCode) return null;
 
@@ -92,6 +107,7 @@ public class FaqBrowserClient : IFaqBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.DeleteAsync($"faq/{id}", cancellationToken: ct);
             return response.IsSuccessStatusCode;
         }

TaxBaik.Web/Services/TaxFilingBrowserClient.cs

@@ -1,5 +1,6 @@
 namespace TaxBaik.Web.Services;
 
+using System.Net.Http;
 using System.Net.Http.Json;
 using TaxBaik.Domain.Entities;
 
@@ -20,17 +21,28 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
 {
     private readonly HttpClient _http;
     private readonly ILogger<TaxFilingBrowserClient> _logger;
+    private readonly ITokenStore _tokenStore;
 
-    public TaxFilingBrowserClient(HttpClient http, ILogger<TaxFilingBrowserClient> logger)
+    public TaxFilingBrowserClient(HttpClient http, ILogger<TaxFilingBrowserClient> logger, ITokenStore tokenStore)
     {
         _http = http;
         _logger = logger;
+        _tokenStore = tokenStore;
+    }
+
+    private void EnsureAuthHeader()
+    {
+        if (!string.IsNullOrEmpty(_tokenStore.AccessToken) && !_http.DefaultRequestHeaders.Contains("Authorization"))
+        {
+            _http.DefaultRequestHeaders.Authorization = new("Bearer", _tokenStore.AccessToken);
+        }
     }
 
     public async Task<IEnumerable<TaxFiling>> GetUpcomingAsync(int daysAhead = 30, CancellationToken ct = default)
     {
         try
         {
+            EnsureAuthHeader();
             var result = await _http.GetFromJsonAsync<TaxFilingListResponse>(
                 $"tax-filing/upcoming?daysAhead={daysAhead}", cancellationToken: ct);
             return result?.Data ?? [];
@@ -46,6 +58,7 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var result = await _http.GetFromJsonAsync<TaxFilingListResponse>(
                 $"tax-filing/client/{clientId}", cancellationToken: ct);
             return result?.Data ?? [];
@@ -61,6 +74,7 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             return await _http.GetFromJsonAsync<TaxFiling>(
                 $"tax-filing/{id}", cancellationToken: ct);
         }
@@ -75,6 +89,7 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PostAsJsonAsync("tax-filing", filing, cancellationToken: ct);
             if (!response.IsSuccessStatusCode)
                 return null;
@@ -95,6 +110,7 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.PutAsJsonAsync($"tax-filing/{id}", filing, cancellationToken: ct);
             if (!response.IsSuccessStatusCode)
                 return null;
@@ -115,6 +131,7 @@ public class TaxFilingBrowserClient : ITaxFilingBrowserClient
     {
         try
         {
+            EnsureAuthHeader();
             var response = await _http.DeleteAsync($"tax-filing/{id}", cancellationToken: ct);
             return response.IsSuccessStatusCode;
         }

TaxBaik.Web/Services/TelegramNotificationService.cs

@@ -0,0 +1,76 @@
+namespace TaxBaik.Web.Services;
+
+using System.Net.Http.Json;
+
+/// <summary>
+/// Telegram Bot 알림 서비스
+/// 중요 로깅 및 오류를 Telegram으로 전송
+/// </summary>
+public interface ITelegramNotificationService
+{
+    Task SendMessageAsync(string message, CancellationToken ct = default);
+    Task SendErrorAsync(string title, string details, CancellationToken ct = default);
+    Task SendInfoAsync(string title, string message, CancellationToken ct = default);
+}
+
+public class TelegramNotificationService : ITelegramNotificationService
+{
+    private readonly HttpClient _httpClient;
+    private readonly ILogger<TelegramNotificationService> _logger;
+    private readonly string _botToken;
+    private readonly string _chatId;
+    private const string TelegramApiUrl = "https://api.telegram.org";
+
+    public TelegramNotificationService(
+        HttpClient httpClient,
+        ILogger<TelegramNotificationService> logger,
+        IConfiguration config)
+    {
+        _httpClient = httpClient;
+        _logger = logger;
+        _botToken = config["Telegram:BotToken"] ?? "";
+        _chatId = config["Telegram:ChatId"] ?? "";
+    }
+
+    public async Task SendMessageAsync(string message, CancellationToken ct = default)
+    {
+        if (string.IsNullOrEmpty(_botToken) || string.IsNullOrEmpty(_chatId))
+        {
+            _logger.LogWarning("Telegram credentials not configured");
+            return;
+        }
+
+        try
+        {
+            var url = $"{TelegramApiUrl}/bot{_botToken}/sendMessage";
+            var payload = new
+            {
+                chat_id = _chatId,
+                text = message,
+                parse_mode = "HTML"
+            };
+
+            var response = await _httpClient.PostAsJsonAsync(url, payload, cancellationToken: ct);
+            if (!response.IsSuccessStatusCode)
+            {
+                _logger.LogError("Failed to send Telegram message: {StatusCode}", response.StatusCode);
+            }
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error sending Telegram message");
+        }
+    }
+
+    public async Task SendErrorAsync(string title, string details, CancellationToken ct = default)
+    {
+        var message = $"<b>❌ {title}</b>\n\n{details}\n\n<i>{DateTime.UtcNow:yyyy-MM-dd HH:mm:ss} UTC</i>";
+        await SendMessageAsync(message, ct);
+    }
+
+    public async Task SendInfoAsync(string title, string message, CancellationToken ct = default)
+    {
+        var text = $"<b>ℹ️ {title}</b>\n\n{message}\n\n<i>{DateTime.UtcNow:yyyy-MM-dd HH:mm:ss} UTC</i>";
+        await SendMessageAsync(text, ct);
+    }
+}