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/platform/templates/postgres.env.j2

@@ -0,0 +1,4 @@
+# Rendered by Ansible — see platform.env.j2 for the pattern.
+POSTGRES_USER=rootnode_platform
+POSTGRES_PASSWORD={{ vault_postgres_password }}
+POSTGRES_DB=rootnode_platform