* feat(install): add one-line Ubuntu installer with CI test and docs
- Add scripts/install-ubuntu.sh: installs Apache/MariaDB/PHP, downloads the
latest stable release, configures the DB + Apache, and optionally sets up
Let's Encrypt SSL (interactive or via env vars).
- Add .github/workflows/install-script-test.yml: runs the installer in a fresh
Ubuntu container and sanity-checks the result.
- Document the installer in INSTALL.md.
- Harden the script: identifier-safe DB_NAME, quote/pipe rejection in
passwords, correct 'Candidate: (none)' PPA detection, keep unix_socket root
auth when no root password is set, select the .zip release asset, and use the
SSL domain as the public hostname for allowedHostnames.
