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/defaults/main.yml

@@ -0,0 +1,21 @@
+---
+# Restrict to security-origin updates only (not a full dist-upgrade).
+unattended_upgrades_security_only: true
+
+# Extra Origins-Pattern lines appended verbatim, for when
+# unattended_upgrades_security_only is turned off later (e.g. to also allow
+# the "-updates" pocket). Left empty by default.
+unattended_upgrades_origins_extra: []
+
+# Reboot handling. Off by default would mean patches needing a reboot never
+# take effect until someone reboots manually; scheduled means an automatic
+# reboot at a fixed, low-traffic time on the days it's actually needed.
+unattended_upgrades_auto_reboot: true
+unattended_upgrades_auto_reboot_time: "03:00"
+
+unattended_upgrades_remove_unused_deps: true
+
+# Set to a mail address (and ensure a working mail transport is configured
+# on the host) to get notified on failure. Empty disables mail entirely.
+unattended_upgrades_mail_to: ""
+unattended_upgrades_mail_on_only_error: true