Add Celery worker for GrampsWeb async tasks (GEDCOM import) (STI-91)

GrampsWeb uses Celery for background tasks like GEDCOM imports. Without
a dedicated worker process, imports get queued in Redis but never
processed — resulting in silent failures. Add grampsweb_celery service
using the same image with celery worker command.

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

compose.yml

@@ -235,6 +235,22 @@ services:
       - db
       - redis
 
+  grampsweb_celery:
+    image: ghcr.io/gramps-project/grampsweb:latest
+    restart: unless-stopped
+    environment:
+      - GRAMPSWEB_SECRET_KEY=${GRAMPSWEB_SECRET_KEY:-dev-grampsweb-secret-key-not-for-production}
+      - GRAMPSWEB_TREE=${GRAMPSWEB_TREE:-Stiftung}
+      - GRAMPSWEB_BASE_URL=${GRAMPSWEB_BASE_URL:-https://ahnenforschung.vhtv-stiftung.de}
+      - GRAMPSWEB_CELERY_CONFIG__broker_url=redis://redis:6379/0
+      - GRAMPSWEB_CELERY_CONFIG__result_backend=redis://redis:6379/0
+      - GRAMPSWEB_NEW_DB_BACKEND=sqlite
+    command: celery -A gramps_webapi.celery worker --loglevel=info
+    volumes:
+      - gramps_data:/app/data
+    depends_on:
+      - redis
+
 volumes:
   dbdata:
   gramps_data: