Don't memoize AutoLinkScrubber across format_html calls

The scrubber was memoized on the view helper in b0fa6525 so that
@regexp_timeout_reported would stop scanning remaining text nodes
after a timeout. But memoizing on the helper leaks that flag across
all format_html calls for the entire request — if one comment triggers
a timeout, every subsequent comment on the page loses auto-linking.

Use a fresh scrubber per call. The @regexp_timeout_reported flag still
works within a single document's scrub, which is the intended scope.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/helpers/html_helper.rb

@@ -1,7 +1,6 @@
 module HtmlHelper
   def format_html(html)
-    @auto_link_scrubber ||= AutoLinkScrubber.new
-    Loofah::HTML5::DocumentFragment.parse(html).scrub!(@auto_link_scrubber).to_html.html_safe
+    Loofah::HTML5::DocumentFragment.parse(html).scrub!(AutoLinkScrubber.new).to_html.html_safe
   end
 
   def card_html_title(card)