docs(website): serve Umami first-party from umami.relplay.org

Brave Shields (and similar blockers) block Umami only as a third-party
request, so point the tracker at umami.relplay.org — a proxied CNAME to
the Umami server that's first-party to relplay.org — instead of the
server's own host. Same Website ID; the data endpoint follows the script
host, so no other change. Privacy page updated to name the new host, and
a comment in astro.config.mjs records why it's a relplay.org subdomain.

website/src/content/docs/privacy.md

@@ -11,7 +11,7 @@ visitors. We keep the analytics deliberately minimal.
 
 We measure aggregate usage with **[Umami](https://umami.is/)**, a
 privacy-friendly analytics tool that we **host ourselves** (on
-`umami.oliversturm.com`). Nothing is sent to a third-party advertising
+`umami.relplay.org`). Nothing is sent to a third-party advertising
 network.
 
 For each page view we record: