fix: restore Sitemap as Razor Page for search engine compatibility

Problem:
- SitemapEndpoint (FastEndpoints) creates /api/sitemap.xml
- robots.txt references /taxbaik/sitemap.xml
- Path mismatch breaks search engine crawling

Solution:
- Restore Sitemap.cshtml (Razor Page)
- Restore Sitemap.cshtml.cs (PageModel with BlogService)
- Remove SitemapEndpoint (FastEndpoints)
- Proper XML Content-Type handling
- Exact path match: /taxbaik/sitemap.xml

Why Razor Page?
- Razor Pages handle exact @page routes better
- Search engines know standard sitemap.xml paths
- No /api prefix routing conflicts
- Direct SSR rendering without endpoint routing

Verification for Google/Naver:
✓ /taxbaik/sitemap.xml (exact match with robots.txt)
✓ Content-Type: application/xml
✓ Valid XML structure
✓ Dynamic blog posts included

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

src/TaxBaik.Web/Pages/Sitemap.cshtml.cs

@@ -0,0 +1,43 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using TaxBaik.Application.Services;
+
+namespace TaxBaik.Web.Pages;
+
+public class SitemapModel : PageModel
+{
+    private readonly BlogService _blogService;
+
+    public List<string> Urls { get; set; } = [];
+
+    public SitemapModel(BlogService blogService)
+    {
+        _blogService = blogService;
+    }
+
+    public async Task OnGetAsync()
+    {
+        var baseUrl = "https://www.taxbaik.com/taxbaik";
+
+        // 정적 페이지
+        Urls.AddRange(new[]
+        {
+            $"{baseUrl}",
+            $"{baseUrl}/about",
+            $"{baseUrl}/services",
+            $"{baseUrl}/contact",
+            $"{baseUrl}/privacy",
+            $"{baseUrl}/terms",
+            $"{baseUrl}/blog",
+            $"{baseUrl}/faq",
+            $"{baseUrl}/announcement",
+            $"{baseUrl}/inquiry"
+        });
+
+        // 동적 블로그 포스트
+        var (posts, _) = await _blogService.GetPublishedPagedAsync(1, 1000, categoryId: null, ct: default);
+        foreach (var post in posts)
+        {
+            Urls.Add($"{baseUrl}/blog/{post.Slug}");
+        }
+    }
+}