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/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;
         }