fix(admin): strip YAML frontmatter before mistune in _rebuild_article()

Fixes a bug where manual article previews rendered raw frontmatter
(title:, slug:, etc.) as visible text. Now strips the --- block using
the existing _FRONTMATTER_RE before passing the body to mistune.html().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/padelnomics/admin/routes.py

@@ -2769,7 +2769,10 @@ async def _rebuild_article(article_id: int):
         md_path = Path("data/content/articles") / f"{article['slug']}.md"
         if not md_path.exists():
             return
-        body_html = mistune.html(md_path.read_text())
+        raw = md_path.read_text()
+        m = _FRONTMATTER_RE.match(raw)
+        body = raw[m.end():] if m else raw
+        body_html = mistune.html(body)
         lang = article.get("language", "en") if hasattr(article, "get") else "en"
         body_html = await bake_scenario_cards(body_html, lang=lang)
         body_html = await bake_product_cards(body_html, lang=lang)