feat(artists): sort control on the roster

Add a Sort dropdown to /artists next to the filter: Recently followed
(default, createdAt desc — the prior order), Name A–Z, Most in library,
Most monitored, Most releases. Client-side sort of the already-fetched
roster; the filter box and "In library, not followed" section are
unchanged. Exposes WatchedArtist.createdAt on /api/artists for the sort.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/api/artists/route.ts

@@ -38,6 +38,7 @@ export async function GET(request: Request) {
         id: a.id,
         mbid: a.mbid,
         name: a.name,
+        createdAt: a.createdAt,
         autoMonitorFuture: a.autoMonitorFuture,
         showAllTypes: a.showAllTypes,
         releaseCount: visible.length,