feat: add RSS/Atom feed support - fix Razor XML rendering

Changes:
- Rss.cshtml: Fixed Razor/XML syntax by using StringBuilder
- Feed.cshtml: Alias for /feed.xml
- Both pages use RssModel (BlogService)
- robots.txt: Added feed references

Fix:
- Removed @page duplicate directive
- Used StringBuilder for proper XML generation
- Avoided Razor XML tag nesting issues
- Both /rss.xml and /feed.xml now available

URLs:
✓ https://www.taxbaik.com/taxbaik/rss.xml
✓ https://www.taxbaik.com/taxbaik/feed.xml

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

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

@@ -0,0 +1,27 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using TaxBaik.Application.Services;
+using TaxBaik.Domain.Entities;
+
+namespace TaxBaik.Web.Pages;
+
+public class RssModel : PageModel
+{
+    private readonly BlogService _blogService;
+
+    public List<BlogPost> Posts { get; set; } = [];
+    public string LastBuildDate { get; set; } = DateTime.UtcNow.ToString("R");
+
+    public RssModel(BlogService blogService)
+    {
+        _blogService = blogService;
+    }
+
+    public async Task OnGetAsync()
+    {
+        // 최근 50개 블로그 포스트 (RSS는 일반적으로 최신 기사만 포함)
+        var (posts, _) = await _blogService.GetPublishedPagedAsync(1, 50, categoryId: null, ct: default);
+        Posts = posts.OrderByDescending(p => p.PublishedAt).ToList();
+
+        LastBuildDate = Posts.FirstOrDefault()?.PublishedAt?.ToString("R") ?? DateTime.UtcNow.ToString("R");
+    }
+}