add simple A/B testing with @ab_test decorator and Umami data-tag

Assigns visitors to experiment variants via cookie (30-day), sets g.ab_variant
and g.ab_tag, and conditionally adds data-tag to the Umami script tag so all
pageviews and events are automatically tagged in the Umami dashboard.

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

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ## [Unreleased]
 
 ### Added
+- Simple A/B testing with `@ab_test` decorator and Umami `data-tag` integration
 - SEO defaults in `base.html`: canonical, og:url, og:type, og:image (logo fallback), og:title, og:description, twitter:card — every page gets these automatically, child templates override as needed
 - `robots.txt` route: disallows `/admin/`, `/auth/`, `/dashboard/`, `/billing/`, `/directory/results`; includes Sitemap reference
 - Meta descriptions and OG tags on all public pages: about, terms, privacy, pricing, features, suppliers, directory, supplier detail