(feature) Make the native installer interactive

Add an interactive setup flow to the systemd installer with defaults
for install paths, service identity, backup storage, bind address,
allowed hosts, CSRF origins, OS package installation, MariaDB support,
nginx setup, and first superuser creation.

Keep scripted installs supported through non-interactive mode, existing
overrides, environment variables, and explicit superuser flags.

Print a user-facing completion summary with the control panel URL,
Self Check reminder, first setup steps, and useful service log commands.

docs/development.md

@@ -62,6 +62,22 @@ pobsync discover-snapshots --host <host>
 pobsync retention <host>
 ```
 
+## Installer Development
+
+The native installer is interactive by default when stdin is a terminal. It should keep every prompt backed by a command
+line flag or environment variable so production installs remain scriptable.
+
+Useful modes:
+
+```
+sudo scripts/install-systemd
+sudo scripts/install-systemd --non-interactive
+sudo scripts/install-systemd --create-superuser --superuser-username admin
+```
+
+The installer should print a short completion summary with the control panel URL, Self Check reminder, and service log
+commands. Keep that output user-facing rather than developer-facing.
+
 ## Migration Helpers
 
 Import existing legacy YAML configs: