feat(ansible): idempotent role-based host config for both production VMs

common/firewall/docker-tls/platform/lab-host/monitoring-agent/backup roles,
orchestrated by site.yml with the ordering that matters (firewall/TLS land
before the services that depend on them). Ansible Vault for secrets, no new
tool introduced for this project's modest secret surface. Explicit
dev/staging/production environment separation, no secrets committed
(.gitignore covers vault plaintext, TLS keys, Terraform state, *.env).

Validated: ansible-playbook --syntax-check clean.

infra/ansible/roles/backup/templates/backup.env.j2

@@ -0,0 +1,6 @@
+PLATFORM_DB_URL=postgres://rootnode_platform:{{ vault_postgres_password }}@localhost:5432/rootnode_platform
+BACKUP_DIR={{ rootnode_data_root }}/backups
+BACKUP_ENCRYPTION_PASSPHRASE={{ vault_backup_encryption_passphrase }}
+BACKUP_RETENTION_DAYS={{ backup_retention_days }}
+BACKUP_OFFSITE_TARGET={{ backup_offsite_target }}
+METRICS_TEXTFILE_DIR=/var/lib/node-exporter/textfile