The scattered scan-settings JSON routes are superseded by the new
admin-only /api/system/settings pair backed by the SettingsRegistry
(introduced in 885f6d8 / bc47450).
- Rewrite system/settings.md around GET/PUT /api/system/settings:
SettingEntry metadata shape (type, min/max, requires_restart,
category, group, is_default), type-aware validation rules, and the
full tunable-setting catalog (scanner, general, security, api, sync,
performance) with defaults, ranges, and restart requirements
- Note the legacy /api/libraries/scan-settings routes as back-compat
only (they now refresh the registry cache on write)
- Add system/config.md for GET/PUT /api/system/config: raw key/value
system configuration (e.g. base_url), including validation notes and
guidance to prefer the typed settings endpoint for registry keys