Fix GrampsWeb GEDCOM import: share cache volume between web and celery (STI-91)

The web container saves uploaded GEDCOM files to /app/cache/export/,
but the Celery worker has its own isolated /app/cache/ and can't find
the files (FileNotFoundError). Add shared gramps_cache volume mounted
at /app/cache/ on both containers.

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

compose.yml

@@ -231,6 +231,7 @@ services:
           --limit-request-line 8190
     volumes:
       - gramps_data:/app/data
+      - gramps_cache:/app/cache
     depends_on:
       - db
       - redis
@@ -248,10 +249,12 @@ services:
     command: celery -A gramps_webapi.celery worker --loglevel=info
     volumes:
       - gramps_data:/app/data
+      - gramps_cache:/app/cache
     depends_on:
       - redis
 
 volumes:
   dbdata:
   gramps_data:
+  gramps_cache:
   ollama_data: