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/content.config.ts

@@ -21,6 +21,7 @@ const reviews = defineCollection({
     tags: z.array(z.string()).default([]),
     link: z.string().url().optional(),
     linkText: z.string().optional(),
+    amazonLink: z.string().url().optional(),
     description: z.string().optional(),
     source: z.string().optional(),
     fromFind: z.string().optional(),
@@ -54,11 +55,13 @@ const find = defineCollection({
     date: z.coerce.date(),
     link: z.string().url(),
     linkText: z.string().optional(),
+    amazonLink: z.string().url().optional(),
     source: z.string(),
     topics: z.array(z.string()).default([]),
     tags: z.array(z.string()).default([]),
     description: z.string().optional(),
     promotedTo: z.string().optional(),
+    shortlist: z.boolean().default(false),
     editorialTags: z.array(z.string()).default([]),
   }),
 });