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>

compose.yml

@@ -197,7 +197,7 @@ services:
       - GRAMPSWEB_ADMIN_EMAIL=${GRAMPSWEB_ADMIN_EMAIL}
       - GRAMPSWEB_ADMIN_PASSWORD=${GRAMPSWEB_ADMIN_PASSWORD}
       - GRAMPSWEB_TREE=${GRAMPSWEB_TREE:-Stiftung}
-      - GRAMPSWEB_BASE_URL=${GRAMPSWEB_BASE_URL:-/ahnenforschung}
+      - GRAMPSWEB_BASE_URL=${GRAMPSWEB_BASE_URL:-http://localhost:8090}
       - GRAMPSWEB_CELERY_CONFIG__broker_url=redis://redis:6379/0
       - GRAMPSWEB_CELERY_CONFIG__result_backend=redis://redis:6379/0
       - GRAMPSWEB_RATELIMIT_STORAGE_URI=redis://redis:6379/1