Fix nh3 panic: use link_rel param instead of rel in attributes

nh3 manages the rel attribute internally — setting it in
tag_attributes triggers an assertion. Use link_rel parameter instead.

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

blog/bp/blog/ghost/ghost_sync.py

@@ -46,7 +46,7 @@ def _sanitize_html(html: str | None) -> str | None:
         },
         attributes={
             "*": {"class", "id", "style"},
-            "a": {"href", "title", "target", "rel"},
+            "a": {"href", "title", "target"},
             "img": {"src", "alt", "title", "width", "height", "loading"},
             "video": {"src", "controls", "width", "height", "poster"},
             "audio": {"src", "controls"},
@@ -55,6 +55,7 @@ def _sanitize_html(html: str | None) -> str | None:
             "td": {"colspan", "rowspan"},
             "th": {"colspan", "rowspan"},
         },
+        link_rel="noopener noreferrer",
         url_schemes={"http", "https", "mailto"},
     )