site: hero auto-discovery, pick.json hero selection, amazonLink + shortlist fields

- src/lib/hero.ts: review heroes are now auto-discovered via import.meta.glob
  over reviews/*/hero.<ext>; only legacy non-hero-named assets remain as
  explicit overrides. New reviews need zero registration.
- Pick of the day moves from a hardcoded HERO_SLUG in index.astro to
  src/data/pick.json (content edit, not code edit); dev set-pick API and
  the review-page button now write pick.json. Fallback: latest review.
- Schemas: finds gain shortlist (bool) + amazonLink; reviews gain amazonLink.
- Review + find pages render an 'Also on Amazon' affiliate link when
  amazonLink is set, with shared disclosure handling.
- Both hero globs now include .avif (two June finds ship avif heroes).

src/pages/reviews/[...slug].astro

@@ -39,6 +39,9 @@ const linkLabel =
   (review.data.linkText ?? new URL(review.data.link).hostname.replace(/^www\./, ''));
 
 const external = review.data.link ? resolveExternalLink(review.data.link) : undefined;
+const amazon = review.data.amazonLink
+  ? resolveExternalLink(review.data.amazonLink)
+  : undefined;
 
 const fromFindItem = review.data.fromFind
   ? (await getCollection('find')).find((f) => f.id === review.data.fromFind)
@@ -111,7 +114,20 @@ const categoryLabel =
       </p>
     )}
 
-    {external?.isAffiliate && (
+    {amazon && (
+      <p class="external">
+        <a
+          href={amazon.href}
+          target="_blank"
+          rel={amazon.isAffiliate ? AFFILIATE_REL : 'noopener noreferrer'}
+        >
+          Also on Amazon ↗
+        </a>
+        {amazon.isAffiliate && <span class="aff-badge">Affiliate</span>}
+      </p>
+    )}
+
+    {(external?.isAffiliate || amazon?.isAffiliate) && (
       <p class="aff-disclosure">{AFFILIATE_DISCLOSURE}</p>
     )}
 
@@ -121,7 +137,7 @@ const categoryLabel =
           label="Make today's pick"
           action="set-pick"
           slug={review.id}
-          confirm={`This will set ${review.data.name} as the homepage hero (rewrites HERO_SLUG in src/pages/index.astro).`}
+          confirm={`This will set ${review.data.name} as the homepage hero (writes src/data/pick.json).`}
         />
       </div>
     )}