Align GrampsWeb setup with official docker-compose (STI-91)

Replace custom GrampsWeb config with the official setup:
- Use YAML anchor (&grampsweb / <<: *grampsweb) for DRY config
- Add all 8 official volumes: users, indexdir, thumbnail_cache, cache,
  secret, grampsdb, media, tmp
- Remove custom admin creation command (use GrampsWeb's built-in flow)
- Remove GRAMPSWEB_SECRET_KEY (managed by gramps_secret volume)
- Celery worker inherits full config via YAML merge
- Shared /tmp volume between web and celery for file transfers

Previous setup only had 2 volumes (data, cache), causing data loss
on container restart and GEDCOM import failures.

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

env-template.txt

@@ -59,10 +59,7 @@ GRAMPS_USERNAME=Stiftung
 GRAMPS_PASSWORD=your-gramps-password-here
 GRAMPS_API_TOKEN=
 
-# GrampsWeb Container Konfiguration
-GRAMPSWEB_SECRET_KEY=your-grampsweb-secret-key-here
-GRAMPSWEB_ADMIN_EMAIL=admin@vhtv-stiftung.de
-GRAMPSWEB_ADMIN_PASSWORD=your-grampsweb-admin-password-here
+# GrampsWeb Container Konfiguration (aligned with official docker-compose)
 GRAMPSWEB_TREE=Stiftung
 # Full external URL (used by GrampsWeb for email links, OIDC, etc.)
 GRAMPSWEB_BASE_URL=https://ahnenforschung.vhtv-stiftung.de