style(api): drop the em dash from the sitemap url docstring

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app.py

@@ -109,7 +109,7 @@ def _has_publishable_data(row) -> bool:
 
 
 def _url_element(loc: str, lastmod: str | None = None) -> str:
-    """One <url> entry. No priority or changefreq — Google ignores both."""
+    """One <url> entry. No priority or changefreq, Google ignores both."""
     body = f"<loc>{loc}</loc>"
     if lastmod:
         body += f"<lastmod>{lastmod}</lastmod>"