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/platform.env.j2

@@ -0,0 +1,18 @@
+# Rendered by Ansible from infra/environments/{{ env }}/platform.env.template's shape —
+# real values come from group_vars/{{ env }}.yml + the encrypted vault file. Do not hand-edit
+# on the host; re-run the platform role instead.
+
+# service names, not localhost — production runs these on a named Docker bridge network
+# (rn-platform-internal), not network_mode: host like the dev compose file does.
+PLATFORM_DB_URL=postgres://rootnode_platform:{{ vault_postgres_password }}@postgres:5432/rootnode_platform
+PROVISIONER_URL=http://provisioner:4100
+PROVISIONER_SHARED_SECRET={{ vault_provisioner_shared_secret }}
+PLATFORM_API_PORT=4300
+
+STUDENT_SESSION_TTL_HOURS=8
+STUDENT_SESSION_IDLE_HOURS=2
+INSTRUCTOR_SESSION_TTL_HOURS=12
+INSTRUCTOR_SESSION_IDLE_HOURS=2
+
+COOKIE_SECURE=1
+TRUST_PROXY=1