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/global_defaults/tasks/system.yml

@@ -30,7 +30,7 @@
     that:
       - item is mapping
       - item.name is defined and (item.name | string | length) > 0
-      - item.keys is not defined or (item.keys is iterable and item.keys is not string)
+      - item['keys'] is not defined or (item['keys'] is iterable and item['keys'] is not string)
     fail_msg: "Each system.users[] entry must be a dict with 'name'; 'keys' must be a list."
     quiet: true
   loop: "{{ system.users }}"