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/README.md

@@ -0,0 +1,9 @@
+# unattended_upgrades
+
+Installs and configures `unattended-upgrades` for automatic security
+patching, separate from `update-packages.yml` (which handles deliberate,
+scheduled full-package updates — see the repo README). Security-only by
+default. Reboots when required, at a fixed scheduled time (default 03:00),
+rather than never or immediately — see `defaults/main.yml` to change this.
+Mail-on-failure is supported but disabled by default since no mail
+transport is configured on these hosts yet.