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>

justfile

@@ -42,6 +42,10 @@ full limit="all":
 server:
     just run server.yml server
 
+# Run the deliberate fleet package-update playbook, e.g. `just update kitchenpi`
+update limit="all":
+    just run update-packages.yml {{limit}}
+
 # List installed Galaxy collections
 collections:
     {{venv_bin}}/ansible-galaxy collection list