Fix remaining ansible-lint violations: file permissions, var-naming, package pinning

- risky-file-permissions (32): add explicit mode: to copy/template/file tasks,
  matching the umask-derived permissions they already had (0644 for configs
  and systemd units, 0755 for created directories) — no functional change.
- var-naming (28): prefix role-scoped vars with their role name across
  pi_dhtsensor, pi_dhtsensor_circuitpython, pi_shairport, pi_squeezelite,
  pi_squeezelite_custom, pi_sispmctl, pi_standard_setup, and pi_sysdweb's
  sysdweb_name (shared by 9 consuming roles). Updated every dependent
  template, task reference, and matching inventory.yml override, and
  verified resolved values with ansible-inventory before/after.
- Fixes a latent bug found while renaming: pi_standard_setup's "Get/Change
  WiFi country" tasks reused the name wifi_country for both the role default
  and a register, so the register silently clobbered the default before
  do_wifi_country ever read it. Split into distinct names so the intended
  default value is used.
- package-latest (2): pin docker-ce/docker-compose-plugin installs in
  server_debian_docker to state: present.
- no-handler (1): move pi_lirc's "Reboot if boot overlay changed" into a
  proper handler notified by the boot-overlay task.

ansible-lint now passes clean at the production profile.

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

roles/pi_standard_setup/README.md

@@ -5,6 +5,7 @@ Pi model, adds a root SSH key, disables the SSH locale-forwarding warning,
 optionally configures wifi and the hostname, and rotates the default
 `pi`/`raspberry` credentials via the `keepass` lookup plugin.
 
-**Key vars:** `wifi_ssid`, `new_hostname`, `timezone`, `wifi_country`,
+**Key vars:** `pi_standard_setup_wifi_ssid`, `pi_standard_setup_new_hostname`,
+`pi_standard_setup_timezone`, `pi_standard_setup_wifi_country`,
 `ansible_ssh_pass` (the OS-default password, used only to reach a
 freshly-flashed Pi for the first time before its password is rotated)