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/tasks/main.yml

@@ -0,0 +1,85 @@
+---
+# rn-platform-01: Docker Engine, directory structure, rendered compose file + env files,
+# Postgres tuning overlay, reverse proxy config, and the systemd unit that runs it all.
+# Docker TLS material and the host firewall are separate roles (docker-tls, firewall) run
+# before this one in site.yml — this role assumes both are already in place.
+
+- name: Install Docker Engine
+  ansible.builtin.import_tasks: "{{ role_path }}/../common/tasks/docker-engine.yml"
+
+- name: Create data/config directories
+  ansible.builtin.file:
+    path: "{{ item }}"
+    state: directory
+    owner: root
+    group: docker
+    mode: "0750"
+  loop:
+    - "{{ rootnode_data_root }}/postgres"
+    - "{{ rootnode_data_root }}/backups"
+    - /opt/rootnode/platform
+    - /opt/rootnode/platform/postgresql.conf.d
+    - /etc/rootnode/provisioner-config
+
+- name: Deploy Postgres tuning overlay
+  ansible.builtin.copy:
+    src: "{{ role_path }}/../../../docker/postgres/postgresql.conf.d/00-rootnode-baseline.conf"
+    dest: /opt/rootnode/platform/postgresql.conf.d/00-rootnode-baseline.conf
+    mode: "0644"
+  notify: restart rootnode-platform
+
+- name: Deploy provisioner-config overlay ({{ env }})
+  ansible.builtin.copy:
+    src: "{{ role_path }}/../../../../environments/{{ env }}/provisioner-config/"
+    dest: /etc/rootnode/provisioner-config/
+    mode: "0644"
+  notify: restart rootnode-platform
+
+- name: Deploy per-service env files (from Ansible Vault-backed variables — see group_vars)
+  ansible.builtin.template:
+    src: "{{ item }}.env.j2"
+    dest: "/etc/rootnode/{{ item }}.env"
+    owner: root
+    group: root
+    mode: "0600"
+  loop:
+    - platform
+    - provisioner
+    - gateway
+    - postgres
+  notify: restart rootnode-platform
+
+- name: Render production docker-compose.yml
+  ansible.builtin.template:
+    src: "{{ role_path }}/../../../docker/platform/docker-compose.production.yml"
+    dest: /opt/rootnode/platform/docker-compose.yml
+    owner: root
+    group: root
+    mode: "0640"
+  notify: restart rootnode-platform
+
+- name: Render Caddyfile
+  ansible.builtin.template:
+    src: "{{ role_path }}/../../../docker/platform/Caddyfile.j2"
+    dest: /opt/rootnode/platform/Caddyfile
+    owner: root
+    group: root
+    mode: "0644"
+  notify: restart rootnode-platform
+
+- name: Deploy systemd unit
+  ansible.builtin.template:
+    src: rootnode-platform.service.j2
+    dest: /etc/systemd/system/rootnode-platform.service
+    mode: "0644"
+  notify: restart rootnode-platform
+
+# The `backup` role (applied after this one in site.yml) deploys the backup script and cron —
+# kept separate since it also has to run correctly on a host that ISN'T freshly deployed.
+
+- name: Enable and start rootnode-platform
+  ansible.builtin.systemd:
+    name: rootnode-platform
+    enabled: true
+    state: started
+    daemon_reload: true