fix(preview): guard want() on res.ok; docs: preview in README + artists-search TODO

- preview-client want() no longer flips the Monitored badge when the POST fails
  (deferred item #1 from the discovery-preview whole-branch review).
- README: discovery section notes the artist preview page.
- Plan Notes + project-state: TODO to reuse the preview page from the /artists
  live search (wire artists-client.tsx hits to /discover/artist/[mbid]).

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

web/src/app/discover/artist/[mbid]/preview-client.tsx

@@ -65,7 +65,7 @@ export function PreviewClient({ mbid, initialName }: { mbid: string; initialName
   }
 
   async function want(r: Release) {
-    await fetch("/api/discover/want", {
+    const res = await fetch("/api/discover/want", {
       method: "POST",
       headers: { "content-type": "application/json" },
       body: JSON.stringify({
@@ -78,6 +78,7 @@ export function PreviewClient({ mbid, initialName }: { mbid: string; initialName
         firstReleaseDate: r.firstReleaseDate,
       }),
     });
+    if (!res.ok) return; // don't flip the badge if the want failed
     setData((d) =>
       d ? { ...d, releases: d.releases.map((x) => (x.rgMbid === r.rgMbid ? { ...x, monitored: true } : x)) } : d,
     );