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/dl/tasks/main.yml

@@ -24,6 +24,7 @@
     owner: root
     group: root
     mode: "0644"
+  when: dl_sftp_authorized_keys | default('') | length > 0
   notify: Restart dl
   tags: dl
 
@@ -52,6 +53,7 @@
     rule: allow
     port: "{{ dl_sftp_port }}"
     proto: tcp
+  when: dl_sftp_authorized_keys | default('') | length > 0
   tags: dl
 
 - name: Start dl stack