refactor(users): migrate system.user to system.users[] for multi-user support

Replace the single-user system.user dict with a system.users list to
support multiple user accounts. Update all roles, templates, examples,
validation, and documentation to use the new format. Remove redundant
post-normalization type checks from validation.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roles/virtualization/tasks/proxmox.yml

@@ -34,8 +34,8 @@
     api_host: "{{ hypervisor_cfg.url }}"
     api_user: "{{ hypervisor_cfg.username }}"
     api_password: "{{ hypervisor_cfg.password }}"
-    ciuser: "{{ system_cfg.user.name }}"
-    cipassword: "{{ system_cfg.user.password }}"
+    ciuser: "{{ system_cfg.users[0].name }}"
+    cipassword: "{{ system_cfg.users[0].password }}"
     ciupgrade: false
     node: "{{ hypervisor_cfg.host }}"
     vmid: "{{ system_cfg.id }}"