Add deliberate update playbook and unattended-upgrades role

Regular playbooks now use state: present, so they no longer upgrade
packages as a side effect. This adds two separate, explicit mechanisms
to keep the fleet patched instead:

- update-packages.yml: ad hoc / to-be-scheduled fleet-wide upgrade
  (safe by default, dist available via -e), plus
  update-packages-pinned-example.yml as a template for pinning or
  bumping a single package outside that.
- roles/unattended_upgrades: automatic security-only patching via
  unattended-upgrades, with a scheduled reboot window and mail
  left disabled pending a configured MTA. Applied to every host in
  full.yml and server.yml.

Also removes a leftover `upgrade: yes` apt task from pi_standard_setup
and server_basic_environment that was still doing a full upgrade on
every routine run, defeating the point of the state: present switch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

roles/unattended_upgrades/tasks/main.yml

@@ -0,0 +1,31 @@
+---
+- name: Install unattended-upgrades
+  ansible.builtin.apt:
+    name: unattended-upgrades
+    state: present
+    update_cache: true
+
+- name: Configure unattended-upgrades behavior
+  ansible.builtin.template:
+    src: 50unattended-upgrades.j2
+    dest: /etc/apt/apt.conf.d/50unattended-upgrades
+    owner: root
+    group: root
+    mode: "0644"
+
+- name: Enable periodic apt update / unattended-upgrade timers
+  ansible.builtin.template:
+    src: 20auto-upgrades.j2
+    dest: /etc/apt/apt.conf.d/20auto-upgrades
+    owner: root
+    group: root
+    mode: "0644"
+
+- name: Ensure apt daily timers are enabled and running
+  ansible.builtin.systemd:
+    name: "{{ item }}"
+    enabled: true
+    state: started
+  loop:
+    - apt-daily.timer
+    - apt-daily-upgrade.timer