Fix GrampsWeb subpath: use nginx sub_filter for <base href> rewrite (STI-93)

GrampsWeb's SPA has <base href="/"> hardcoded at build time. The
GRAMPSWEB_BASE_URL env var is a full URL for API/OIDC, not a path prefix.
This means assets always load from root, hitting Django instead of GrampsWeb.

Fix: nginx sub_filter rewrites <base href="/"> to <base href="/ahnenforschung/">
so the browser resolves all SPA assets under the correct subpath.

Also revert BASE_URL default to a proper URL (not a path).

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

deploy-production/nginx.conf

@@ -83,6 +83,8 @@ server {
     }
 
     # GrampsWeb Ahnenforschung
+    # GrampsWeb SPA has <base href="/"> hardcoded — sub_filter rewrites it
+    # so asset URLs resolve under /ahnenforschung/ instead of /
     location /ahnenforschung/ {
         proxy_pass http://127.0.0.1:8090/;
         proxy_set_header Host $host;
@@ -91,7 +93,14 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
         proxy_set_header X-Script-Name /ahnenforschung;
 
-        # WebSocket support (GrampsWeb uses SSE/WS for real-time updates)
+        # Rewrite <base href="/"> to <base href="/ahnenforschung/">
+        # so the SPA loads JS/CSS from the correct subpath
+        proxy_set_header Accept-Encoding "";
+        sub_filter '<base href="/">' '<base href="/ahnenforschung/">';
+        sub_filter_once on;
+        sub_filter_types text/html;
+
+        # WebSocket support
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection "upgrade";