fix: Add media_files volume to worker service (STI-84)

Celery worker was missing the media_files:/app/media volume mount,
causing DMS files saved by background tasks (email attachments,
Bestätigungsschreiben PDFs) to land in ephemeral container storage
instead of the persistent named volume. After any container restart,
these files were lost while DB records remained → Http404.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy-production/docker-compose.prod.yml

@@ -79,6 +79,8 @@ services:
     depends_on:
       - redis
       - db
+    volumes:
+      - media_files:/app/media
     command: ["celery", "-A", "core", "worker", "-l", "info"]
 
   beat: