tracking(bf): mark the offer CTAs on the two Black Friday pages as Buy

The two Black Friday landing pages carried 0 Buy marks, so the page we are
about to advertise reported nothing. Buy is a CLICK PROXY by contract (it marks
a click on the offer CTA, not a completed purchase); the real purchase is fired
on the checkout host (20.odoo4projects.com) by odoo-dev. Both CTAs on
/odoo-black-friday/ and the BF-block link on /odoo-hosting-offers/ now carry
plausible-event-name=Buy, each labelled in-page as the proxy it is.

No Google Ads tag added: see handoff 2026-11-01-webmaster-to-olchi-ads-tag-and-buy-proxy.md
(privacy.html discloses no advertising cookies and the site has no consent gate).

odoo-hosting-offers/index.html

@@ -484,7 +484,9 @@
           <h2>Black Friday and Cyber Monday 2026</h2>
           <p class="bf-hub-line">This November we take $100 off your first year of hosting. The window is 16–30 November 2026, and it ends on Cyber Monday.</p>
           <!-- 2026-11-01, dated for removal: the offer and the code are revealed on /odoo-black-friday/ only. Remove this link with the BF block after 30 November 2026. -->
-          <p class="bf-hub-link"><a href="/odoo-black-friday/">See the Black Friday offer</a></p>
+          <!-- Buy = CLICK PROXY (offer-CTA click), NOT a sale. The real purchase event is fired on the
+               checkout host (20.odoo4projects.com) by odoo-dev. See wiki/concepts/conversion-tracking-contract. -->
+          <p class="bf-hub-link"><a class="plausible-event-name=Buy" href="/odoo-black-friday/">See the Black Friday offer</a></p>
         </div>
         <!-- BF-BLOCK-END -->
       </div>