feat(deploy): Ansible provisioning for prod hosts (Stage 18)

Idempotent playbooks under deploy/ansible/ prepare both production hosts:
docker-ce + compose plugin, a non-sudo deploy service account holding the CI
deploy key, key-only sshd, default-deny ufw, fail2ban, unattended upgrades and
chrony. The main host also opens 80/443/9443 and creates the external edge
network; the tg host verifies direct Bot API egress (the no-VPN assumption).

The application is deployed separately by the prod-deploy workflow (later
phase), running as the deploy account this playbook provisions.

deploy/ansible/README.md

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+# Prod host provisioning (Stage 18)
+
+Idempotent Ansible that prepares the two production hosts. It installs Docker, a
+non-sudo `deploy` service account, SSH hardening, a default-deny firewall,
+fail2ban, unattended security upgrades and time sync. It does **not** deploy the
+application — that is `.gitea/workflows/prod-deploy.yaml`'s job, running as the
+`deploy` account this playbook creates.
+
+Hosts are referenced by `~/.ssh/config` aliases (`scrabble-main-ops`,
+`scrabble-tg-ops`), so no IPs or key paths live in the repo.
+
+## Prerequisites (controller)
+
+- `ansible` with the bundled collections (`community.general`, `community.docker`,
+  `ansible.posix`).
+- The two hosts reachable as root via the ssh-config aliases, host keys already
+  accepted into `known_hosts` (`host_key_checking = True`).
+
+## One-time: the CI deploy key
+
+The CI prod-deploy workflow logs into the hosts as `deploy` using a dedicated
+key. Generate it once on the controller, authorize its public half via the
+playbook, and store its private half **only** in the Gitea `PROD_SSH_KEY` secret:
+
+```sh
+ssh-keygen -t ed25519 -N '' -C scrabble-ci-deploy \
+  -f ~/.ssh/scrabble_ci_deploy_ed25519
+# private half -> Gitea secret PROD_SSH_KEY (set via API); never commit it
+```
+
+## Run
+
+```sh
+cd deploy/ansible
+ansible-playbook site.yml
+```
+
+The playbook reads the public key from `~/.ssh/scrabble_ci_deploy_ed25519.pub` by
+default; override with `-e deploy_ci_pubkey_path=/path/to/key.pub`. Re-running is
+safe (idempotent) and survives a host resize.
+
+## What each host gets
+
+- **both** (`common`): docker-ce + compose plugin, `daemon.json` (live-restore,
+  10m×3 log rotation), `deploy` user (docker group, no sudo), key-only sshd,
+  `ufw` default-deny incoming + allow SSH, fail2ban sshd jail, unattended
+  upgrades, chrony, `/opt/scrabble/{config,certs,dumps,images}`.
+- **main**: `ufw` opens 80/443/9443; the external `edge` docker network.
+- **tg**: verifies direct `api.telegram.org` egress (the no-VPN assumption).