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)

docs/runbook-provisioning.md

@@ -11,6 +11,7 @@ setup of individual services.
 ### Local tools
 
 - Ansible installed (`pip install ansible`)
+- `passlib` installed (`pip install passlib`) — required for bcrypt/SHA-512 password hashing in templates
 - `openssl` available (for generating secrets)
 
 ### SSH keys