refactor: Phase 5 - JWT token lifecycle (Access + Refresh + Auto-refresh)

**Implementation:**
- AuthService: Split token generation
  * AccessToken: 15 minutes
  * RefreshToken: 7 days (10080 minutes)
  * New: GenerateTokenPair() method
  * New: RefreshAccessTokenAsync() method

- AuthTokenPair: New record (accessToken, refreshToken, expiresIn)

- AuthController: New /api/auth/refresh endpoint
  * POST /api/auth/refresh?refreshToken=...
  * Response: { accessToken, refreshToken, expiresIn }
  * RefreshTokenRequest DTO

- TokenRefreshHandler: New DelegatingHandler
  * Automatic Bearer token injection
  * 401 response handling
  * Auto-refresh with retry
  * localStorage sync (accessToken, refreshToken, tokenExpiry)

- CustomAuthenticationStateProvider: Token storage split
  * Before: auth_token (single)
  * After: accessToken, refreshToken, tokenExpiry
  * LoginAsync signature updated

- Login.razor: Handle token pair
  * LoginResponse: { accessToken, refreshToken, expiresIn }
  * Call new LoginAsync(accessToken, refreshToken, expiresIn)

- Program.cs: TokenRefreshHandler registration
  * AddScoped<TokenRefreshHandler>()
  * AdminDashboardClient pipeline: .AddHttpMessageHandler<TokenRefreshHandler>()

**SOLID Principles:**
✓ S (Single Responsibility): TokenRefreshHandler handles only token refresh
✓ D (Dependency Inversion): DelegatingHandler abstracts HTTP concerns
✓ O (Open/Closed): Token lifetime extension without code changes

**Security Pattern:**
- Short-lived access tokens (15min) reduce theft window
- Refresh tokens (7d) enable persistence without storing secrets
- Automatic refresh is transparent to components

**Flow:**
Blazor → AdminDashboardClient → TokenRefreshHandler (auto-add Bearer)
  → 401 → RefreshTokenAsync() → POST /api/auth/refresh
  → Store new pair → Retry original request

Status: Token lifecycle complete, ready for SignalR integration (Phase 6)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Web/Services/TokenRefreshHandler.cs

@@ -0,0 +1,108 @@
+namespace TaxBaik.Web.Services;
+
+using System.Net;
+using System.Text.Json;
+
+/// <summary>
+/// HTTP 요청 시 자동으로 access token을 추가하고,
+/// 401 응답을 받으면 refresh token으로 새 토큰을 획득한 후 재시도합니다.
+/// SOLID: Single Responsibility - 토큰 갱신 로직만 담당
+/// </summary>
+public class TokenRefreshHandler : DelegatingHandler
+{
+    private readonly ILocalStorageService _localStorage;
+    private readonly ILogger<TokenRefreshHandler> _logger;
+
+    public TokenRefreshHandler(ILocalStorageService localStorage, ILogger<TokenRefreshHandler> logger)
+    {
+        _localStorage = localStorage;
+        _logger = logger;
+    }
+
+    protected override async Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request,
+        CancellationToken cancellationToken)
+    {
+        // 요청에 access token 추가
+        var accessToken = await _localStorage.GetItemAsStringAsync("accessToken");
+        if (!string.IsNullOrEmpty(accessToken))
+        {
+            request.Headers.Authorization = new("Bearer", accessToken);
+        }
+
+        var response = await base.SendAsync(request, cancellationToken);
+
+        // 401 응답이면 토큰 갱신 시도
+        if (response.StatusCode == HttpStatusCode.Unauthorized)
+        {
+            var refreshToken = await _localStorage.GetItemAsStringAsync("refreshToken");
+            if (!string.IsNullOrEmpty(refreshToken))
+            {
+                var newTokenPair = await RefreshTokenAsync(refreshToken, request, cancellationToken);
+                if (newTokenPair != null)
+                {
+                    // 토큰 저장
+                    await _localStorage.SetItemAsStringAsync("accessToken", newTokenPair.AccessToken);
+                    await _localStorage.SetItemAsStringAsync("refreshToken", newTokenPair.RefreshToken);
+                    await _localStorage.SetItemAsStringAsync("tokenExpiry",
+                        DateTime.UtcNow.AddSeconds(newTokenPair.ExpiresIn).Ticks.ToString());
+
+                    // 새 토큰으로 재요청
+                    request.Headers.Authorization = new("Bearer", newTokenPair.AccessToken);
+                    response = await base.SendAsync(request, cancellationToken);
+                }
+                else
+                {
+                    _logger.LogWarning("토큰 갱신 실패 - 로그아웃");
+                    await _localStorage.RemoveItemAsync("accessToken");
+                    await _localStorage.RemoveItemAsync("refreshToken");
+                    await _localStorage.RemoveItemAsync("tokenExpiry");
+                }
+            }
+        }
+
+        return response;
+    }
+
+    private async Task<AuthTokenPair?> RefreshTokenAsync(string refreshToken, HttpRequestMessage originalRequest, CancellationToken ct)
+    {
+        try
+        {
+            // 원래 요청의 호스트 정보 추출
+            var authority = originalRequest.RequestUri?.Authority ?? "localhost:5001";
+            var scheme = originalRequest.RequestUri?.Scheme ?? "http";
+
+            using var httpClient = new HttpClient();
+            var refreshUri = new Uri($"{scheme}://{authority}/taxbaik/api/auth/refresh");
+            var json = JsonSerializer.Serialize(new { refreshToken });
+            var content = new StringContent(json, System.Text.Encoding.UTF8, "application/json");
+
+            var response = await httpClient.PostAsync(refreshUri, content, ct);
+            if (!response.IsSuccessStatusCode)
+            {
+                _logger.LogWarning($"Token refresh failed with status {response.StatusCode}");
+                return null;
+            }
+
+            var responseContent = await response.Content.ReadAsStringAsync(ct);
+            var result = JsonSerializer.Deserialize<AuthTokenResponse>(responseContent,
+                new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+
+            return result != null
+                ? new AuthTokenPair(result.AccessToken, result.RefreshToken, result.ExpiresIn)
+                : null;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Exception during token refresh");
+            return null;
+        }
+    }
+}
+
+internal class AuthTokenResponse
+{
+    public string AccessToken { get; set; } = string.Empty;
+    public string RefreshToken { get; set; } = string.Empty;
+    public int ExpiresIn { get; set; }
+}