Fix GrampsWeb: set BASE_URL default to /ahnenforschung for subpath (STI-93)

The SPA needs GRAMPSWEB_BASE_URL=/ahnenforschung to generate correct
asset URLs when served behind nginx at /ahnenforschung/. Without this,
JS/CSS assets load from / instead of /ahnenforschung/, causing a blank page.

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:-/}
+      - GRAMPSWEB_BASE_URL=${GRAMPSWEB_BASE_URL:-/ahnenforschung}
       - 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

env-template.txt

@@ -64,6 +64,6 @@ GRAMPSWEB_SECRET_KEY=your-grampsweb-secret-key-here
 GRAMPSWEB_ADMIN_EMAIL=admin@vhtv-stiftung.de
 GRAMPSWEB_ADMIN_PASSWORD=your-grampsweb-admin-password-here
 GRAMPSWEB_TREE=Stiftung
-GRAMPSWEB_BASE_URL=/
+GRAMPSWEB_BASE_URL=/ahnenforschung