fix: resolve Jinja2 .keys ambiguity, fastfetch availability, and python interpreter

- Use bracket notation item['keys'] instead of item.keys to avoid
  conflict with Python dict .keys() method
- Remove fastfetch from Debian 12 package list (only available in 13+)
- Set explicit python interpreter path for post-reboot tasks

roles/configuration/tasks/users.yml

@@ -26,7 +26,7 @@
   changed_when: configuration_user_result.rc == 0
 
 - name: Ensure .ssh directory exists
-  when: item.keys | default([]) | length > 0
+  when: item['keys'] | default([]) | length > 0
   ansible.builtin.file:
     path: "/mnt/home/{{ item.name }}/.ssh"
     state: directory