## Why

The inventory currently hardcodes `ansible_user: root`. Many server providers create a non-root user by default (e.g., `ubuntu`, `debian`), and users may prefer to SSH as a non-root user with sudo privileges. The SSH key is pre-configured by the user before running the playbook.

## What Changes

- Change `ansible_user` in inventory to a configurable placeholder instead of hardcoded `root`
- Ensure `ansible.cfg` privilege escalation settings work for both root and non-root users
- Update the `sshd_config` template: when connecting as root, allow `prohibit-password`; when connecting as non-root, the user doesn't need root SSH access at all
- Update README to document SSH user configuration

## Capabilities

### New Capabilities

(none)

### Modified Capabilities
- `server-base`: SSH configuration must work for both root and non-root `ansible_user`

## Impact

- `inventory/hosts.yml` updated with configurable user placeholder
- `roles/base/templates/sshd_config.j2` adjusted for flexibility
- `README.md` updated with SSH user configuration instructions