fix: add Antiforgery cookie configuration for Nginx proxy HTTPS

- Add SameSite=Lax to session cookie
- Add SecurePolicy=SameAsRequest for proxy compatibility
- Explicitly configure Antiforgery cookie with same settings
- Resolves antiforgery token validation failures on HTTPS

This fixes the "required antiforgery cookie is not present" error
that occurs when behind Nginx reverse proxy with HTTPS.

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

src/TaxBaik.Web/Program.cs

@@ -106,9 +106,20 @@ builder.Services.AddSession(options =>
     options.Cookie.HttpOnly = true;
     options.Cookie.IsEssential = true;
     options.Cookie.Name = "TaxBaik.SessionId";
+    options.Cookie.SameSite = Microsoft.AspNetCore.Http.SameSiteMode.Lax;
+    options.Cookie.SecurePolicy = Microsoft.AspNetCore.Http.CookieSecurePolicy.SameAsRequest;
 });
 builder.Services.AddDistributedMemoryCache();
-// TempData는 기본적으로 쿠키 저장소 사용 (여기서 명시적 설정)
+
+// Antiforgery 쿠키 설정 (Nginx 프록시 뒤 HTTPS 지원)
+builder.Services.AddAntiforgery(options =>
+{
+    options.Cookie.HttpOnly = true;
+    options.Cookie.IsEssential = true;
+    options.Cookie.SameSite = Microsoft.AspNetCore.Http.SameSiteMode.Lax;
+    options.Cookie.SecurePolicy = Microsoft.AspNetCore.Http.CookieSecurePolicy.SameAsRequest;
+});
+// TempData는 기본적으로 쿠키 저장소 사용 (위 세션 설정 상속)
 
 // JWT 인증
 var connectionString = builder.Configuration.GetConnectionString("Default")