feat(admin): add cache purge controls to settings

Add `purgeMetadataCache` and `purgeImageCache` procedures that let admins free disk space on demand. Metadata purge removes un-enriched stub titles not in any user's library plus orphaned person records; image purge deletes all cached TMDB files from disk.

Expose both as POST endpoints under `/admin/cache/` via the oRPC contract, implement the core logic in a new `@sofa/core/cache` module, and surface them in a new "Danger Zone" section on the Settings page with individual confirmation dialogs and a combined "Purge all" action.

packages/db/src/helpers.ts

@@ -6,7 +6,9 @@ export {
   gte,
   inArray,
   isNotNull,
+  isNull,
   lt,
+  notInArray,
   or,
   sql,
 } from "drizzle-orm";