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_slint_frontend/tasks/main.yml

@@ -102,7 +102,7 @@
     # codegen-units=1 is a memory risk on 1.8 GB and buys nothing that matters here.
     CARGO_PROFILE_RELEASE_LTO: "false"
     CARGO_PROFILE_RELEASE_CODEGEN_UNITS: "16"
-    PATH: "/root/.cargo/bin:{{ ansible_env.PATH }}"
+    PATH: "/root/.cargo/bin:{{ ansible_facts['env']['PATH'] }}"
   async: 7200
   poll: 30
   changed_when: true