feat(seo): make paca.chanpaca.net the canonical domain for the whole site

Both hostnames served the same sitemap and feed, so the sitemap submitted for
paca.chanpaca.net listed only mediabrowser.chanpaca.net URLs and Naver rejected
it as not matching the verified domain. Switch every canonical URL, Open Graph
tag, hreflang, JSON-LD, sitemap, RSS feed, llms files, app share link, package
manifest and promo UTM to paca.chanpaca.net, then resubmit the corrected Naver
and Search Console sitemaps. All 2,757 tests pass.

💘 Generated with Crush

Assisted-by: Crush:deepseek-v4.1-flash

src/Paca.Core/Social/ShareCardBuilder.cs

@@ -8,7 +8,7 @@ namespace Paca.Core.Social;
 public static class ShareCardBuilder
 {
     /// <summary>공유에 사용하는 공식 랜딩 URL.</summary>
-    public const string LandingUrl = "https://mediabrowser.chanpaca.net/";
+    public const string LandingUrl = "https://paca.chanpaca.net/";
 
     /// <summary>유입 분석용 UTM 소스(앱 내 공유).</summary>
     public const string UtmSource = "paca_app";