Switch GrampsWeb from subpath to subdomain ahnenforschung.vhtv-stiftung.de (STI-91)

The subpath approach (/ahnenforschung/) required ~15 nginx sub_filter
rules to rewrite JS/CSS/HTML paths, causing persistent issues with
font loading, Service Worker, and API routes. GrampsWeb is not designed
for subpath deployment.

Switch to dedicated subdomain where GrampsWeb runs on root /:
- nginx.conf: remove all sub_filter rules, add clean subdomain server block
- base.html: update sidebar link to subdomain URL
- compose.yml: update GRAMPSWEB_BASE_URL default
- env-template.txt: update BASE_URL reference

After deploy: run certbot for ahnenforschung.vhtv-stiftung.de SSL cert,
update .env GRAMPSWEB_BASE_URL, restart containers.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

app/templates/base.html

@@ -689,7 +689,7 @@
                 <i class="fas fa-book-open"></i>
                 <span>Geschichte</span>
             </a>
-            <a class="sidebar-link" href="/ahnenforschung/" target="_blank">
+            <a class="sidebar-link" href="https://ahnenforschung.vhtv-stiftung.de" target="_blank">
                 <i class="fas fa-tree"></i>
                 <span>Ahnenforschung</span>
             </a>