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>

README.md

@@ -27,8 +27,11 @@ Built in three slices:
    aggregates artists similar to the ones you follow into a manual review feed of
    suggested **artists** and **albums** at `/discover`; **Follow** and **Want**
    reuse the slice-2 follow / wanted flows, **Dismiss** is permanent. Plus a
-   web-side seed-search box. No Spotify (its recommendation APIs are dead for new
-   apps).
+   web-side seed-search box. Every suggestion and seed-search result opens a
+   read-only **artist preview** (`/discover/artist/[mbid]`) — discography,
+   per-album tracklists, and "in library" / "monitored" badges — where you can
+   Follow or Want before committing. No Spotify (its recommendation APIs are dead
+   for new apps).
 
 **Operational notes:** the monitor and the discovery sweep are both **off by
 default** — set `monitor.enabled=true` (+ optional `monitor.*` tuning) in the