Move to dedicated regress.atobitcoin.io subdomain

DNS now exists for the subdomain, so retired the podsteadr.atobitcoin.io/regress/
path-prefix arrangement in favor of a clean root deployment on its own
domain: real Let's Encrypt cert (certbot certonly --webroot, same method
as podsteadr's own cert), independent nginx server blocks, no more
ROUTE_PREFIX/VITE_BASE needed for the live deploy (though still supported
and tested for future use elsewhere). Removed the now-dead /regress/
location from podsteadr's own nginx config.

deploy/nginx-podsteadr-regress-path-prefix.conf.historical

@@ -0,0 +1,18 @@
+# Snippet added to podsteadr's existing /etc/nginx/sites-enabled/podsteadr,
+# inside the existing `server { listen 443 ssl; ... }` block, alongside its
+# /blossom/, /player/, /hls/, /whip/ locations.
+#
+# Unlike those locations, this one must NOT strip the /regress/ prefix —
+# proxy_pass has no trailing path component, which tells nginx to forward
+# the original request URI verbatim (prefix included). See README.md
+# "Path-prefix deployment" for why: Regress's NIP-98 login signs the exact
+# URL it calls, prefix included, and the backend (ROUTE_PREFIX=/regress)
+# expects to see that same prefixed path.
+location /regress/ {
+    proxy_pass http://127.0.0.1:8199;
+    proxy_http_version 1.1;
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $scheme;
+}