fix: prevent admin authentication timeout during session

**Issues Resolved:**
1. Access Token lifetime extended 15m → 1h (better UX)
   - Users can browse admin pages for 1 hour without re-login
   - Reasonable balance between security and usability

2. Automatic pre-expiry token refresh
   - GetAuthenticationStateAsync() now checks if token expires in <5min
   - Automatically refreshes before expiry when user is still active
   - Prevents sudden logout during admin work

**Implementation:**
- Added ShouldRefreshToken() to detect imminent expiry (300s window)
- On auth state check, if token expiring soon: trigger refresh via AuthService
- Refresh happens transparently, no user interaction needed
- Maintains 7-day Refresh Token TTL for security

**Behavior:**
- User logs in with 1-hour session
- Every page load/navigation checks token status
- If <5min remaining: auto-refresh (user doesn't notice)
- If refresh fails: graceful logout with warning
- Refresh Token (7 days) allows re-login without password

This provides better UX while maintaining security through
shorter-lived access tokens and automatic renewal.

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

TaxBaik.Web/Services/AuthService.cs

@@ -14,7 +14,7 @@ public class AuthService
     private readonly ILogger<AuthService> _logger;
     private readonly string _jwtSecretKey;
     private readonly string? _passwordResetToken;
-    private readonly int _accessTokenExpirationMinutes = 15;   // Access Token: 15분
+    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)

TaxBaik.Web/Services/CustomAuthenticationStateProvider.cs

@@ -51,13 +51,33 @@ public class CustomAuthenticationStateProvider : AuthenticationStateProvider
                 return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
             }
 
+            // 토큰이 만료되면 로그아웃
             if (_tokenStore.IsAccessTokenExpired())
             {
-                _logger.LogWarning("Access token 만료됨");
+                _logger.LogWarning("Access token 만료됨 - 자동 로그아웃");
                 await LogoutAsync();
                 return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
             }
 
+            // 토큰이 5분 이내로 만료되면 자동 갱신 시도 (사용자 경험 향상)
+            if (!string.IsNullOrEmpty(_tokenStore.RefreshToken) && ShouldRefreshToken())
+            {
+                _logger.LogInformation("토큰 만료 5분 전 - 자동 갱신 시작");
+                var newTokenPair = await _authService.RefreshAccessTokenAsync(_tokenStore.RefreshToken);
+                if (newTokenPair != null)
+                {
+                    await LoginAsync(newTokenPair.AccessToken, newTokenPair.RefreshToken, newTokenPair.ExpiresIn);
+                    _logger.LogInformation("토큰 자동 갱신 성공");
+                    accessToken = newTokenPair.AccessToken;
+                }
+                else
+                {
+                    _logger.LogWarning("토큰 자동 갱신 실패 - 로그아웃");
+                    await LogoutAsync();
+                    return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
+                }
+            }
+
             var principal = _authService.ValidateToken(accessToken);
             if (principal == null)
             {
@@ -91,6 +111,25 @@ public class CustomAuthenticationStateProvider : AuthenticationStateProvider
         NotifyAuthenticationStateChanged(GetAuthenticationStateAsync());
     }
 
+    private bool ShouldRefreshToken()
+    {
+        // 토큰이 5분 이내로 만료되면 갱신 (300초 = 5분)
+        if (_tokenStore.TokenExpiryTicks <= 0)
+            return false;
+
+        const int refreshThresholdSeconds = 300;
+        try
+        {
+            var expiryTime = new DateTime((long)_tokenStore.TokenExpiryTicks, DateTimeKind.Utc);
+            var timeUntilExpiry = expiryTime - DateTime.UtcNow;
+            return timeUntilExpiry.TotalSeconds <= refreshThresholdSeconds && timeUntilExpiry.TotalSeconds > 0;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     public async Task LogoutAsync()
     {
         // TokenStore 초기화