feat: add monthly Docker prune cron Ansible playbook (#29)

Closes #29

Deploys /etc/cron.monthly/docker-prune to all six Docker hosts via
Ansible. Uses 720h (30-day) age filter on containers and images, with
volume pruning exempt for `keep`-labeled volumes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ansible/playbooks/docker-prune.yml

@@ -0,0 +1,55 @@
+---
+# Monthly Docker Prune — Deploy Cleanup Cron to All Docker Hosts
+#
+# Deploys /etc/cron.monthly/docker-prune to each VM running Docker.
+# The script prunes stopped containers, unused images, and orphaned volumes
+# older than 30 days (720h). Volumes labeled `keep` are exempt.
+#
+# Resolves accumulated disk waste from stopped containers and stale images.
+# The `--filter "until=720h"` age gate prevents removing recently-pulled
+# images that haven't started yet. `docker image prune -a` only removes
+# images not referenced by any container (running or stopped), so the
+# age filter adds an extra safety margin.
+#
+# Hosts: VM 106 (docker-home), VM 110 (discord-bots), VM 112 (databases-bots),
+#        VM 115 (docker-sba), VM 116 (docker-home-servers), manticore
+#
+# Controller: LXC 304 (ansible-controller) at 10.10.0.232
+#
+# Usage:
+#   # Dry run (shows what would change, skips writes)
+#   ansible-playbook /opt/ansible/playbooks/docker-prune.yml --check
+#
+#   # Single host
+#   ansible-playbook /opt/ansible/playbooks/docker-prune.yml --limit docker-sba
+#
+#   # All Docker hosts
+#   ansible-playbook /opt/ansible/playbooks/docker-prune.yml
+#
+# To undo: rm /etc/cron.monthly/docker-prune on target hosts
+
+- name: Deploy Docker monthly prune cron to all Docker hosts
+  hosts: docker-home:discord-bots:databases-bots:docker-sba:docker-home-servers:manticore
+  become: true
+
+  tasks:
+    - name: Deploy docker-prune cron script
+      ansible.builtin.copy:
+        dest: /etc/cron.monthly/docker-prune
+        owner: root
+        group: root
+        mode: "0755"
+        content: |
+          #!/bin/bash
+          # Monthly Docker cleanup — deployed by Ansible (issue #29)
+          # Prunes stopped containers, unused images (>30 days), and orphaned volumes.
+          # Volumes labeled `keep` are exempt from volume pruning.
+          set -euo pipefail
+
+          docker container prune -f --filter "until=720h"
+          docker image prune -a -f --filter "until=720h"
+          docker volume prune -f --filter "label!=keep"
+
+    - name: Verify docker-prune script is executable
+      ansible.builtin.command: test -x /etc/cron.monthly/docker-prune
+      changed_when: false