Fix Redis persistence crash-loop and provisioning dependencies

- Disable Redis RDB snapshots (--save "") — Redis is cache-only for
  Nextcloud; snapshot writes failed due to directory permissions and
  blocked all Redis writes, causing background jobs to crash-loop at
  400%+ CPU
- Add passlib as a required local dependency (needed for password_hash
  filter in Ansible templates)
- Make dl role SFTP setup conditional on dl_sftp_authorized_keys being
  set in the vault
- Fix check-vault.sh stdin conflict (pipe + heredoc) by passing vault
  content via environment variable
- Remove dl_sftp_authorized_keys from required_scalars (it is optional)

ansible/roles/nextcloud/templates/docker-compose.yml.j2

@@ -22,7 +22,7 @@ services:
     image: redis:7-alpine
     container_name: nextcloud-redis
     restart: unless-stopped
-    command: --save 60 1 --loglevel warning
+    command: --save "" --loglevel warning
     volumes:
       - {{ nextcloud_data_dir }}/redis:/data
     networks: