feat(settings): add preview_settings for per-extension preview management

Adds a new admin setting `preview_settings` (text/JSON, default `{}`)
in the PREVIEW group. It is the per-extension override layer (order +
disabled IDs) consumed by the new admin UI in alist-web.

The setting is orthogonal to iframe_previews / external_previews —
no migration code, no shadow writes. Backup/restore works across
version boundaries because old clients simply ignore the unknown key.

Companion frontend PR: AlistGo/alist-web#306
Closes #9542