Keep the media-pi roles working on trixie

- pi_hifiberry_amp: the boot config lives at /boot/firmware/config.txt.
- pi_sysdweb: install python3-dbus and python3-systemd from apt, which trixie ships at
  the versions sysdweb needs; bookworm gets the headers pip needs to build dbus-python
  1.4 itself.
- pi_squeezeserver: trixie images have no iptables, and so cannot have the port 80
  redirect that absent.yml removes - skip that step there instead of failing.
- pi_modern_shell_env, pi_slint_frontend: read facts through `ansible_facts` rather than
  the deprecated injected `ansible_*` variables.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/pi_squeezeserver/tasks/absent.yml

@@ -36,7 +36,17 @@
     state: absent
   notify: Restart sysdweb after dropping the LMS entry
 
+- name: Check whether iptables is installed
+  # Trixie images ship nftables only. A host without iptables cannot have the redirect
+  # below (present.yml installs iptables-persistent), so there is nothing to undo.
+  ansible.builtin.command: which iptables
+  register: pi_squeezeserver_iptables
+  changed_when: false
+  failed_when: false
+  check_mode: false # read-only
+
 - name: Undo the port 80 -> 9000 redirect the install added
+  when: pi_squeezeserver_iptables.rc == 0
   ansible.builtin.iptables:
     table: nat
     chain: PREROUTING