search: return all matches (not just 100) + add typeahead autocomplete

- searchLegacyArticles() default limit: 50 → 5000. /search page
    explicitly passes 5000 too. ilike on title/excerpt/author is fast
    enough on the 29k-row archive that returning everything is fine.

  - New API route /api/search/suggest?q=...&limit=8 returns the top
    matching article titles + category + slug, with a 30-second CDN
    cache + 60-second stale-while-revalidate.

  - The search input in the Browse bar now debounces (150ms) and
    fires that API per keystroke once 2+ chars are typed. Matches
    appear in a styled dropdown below the input — click to navigate
    directly to /news/<slug>, or press Enter to fall through to the
    full /search page. "See all results for …" link in the dropdown
    footer.

  - Keyboard: Escape closes the dropdown; aria-autocomplete + role
    listbox/option for accessibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/articles/legacy-source.ts

@@ -314,9 +314,13 @@ export async function getLegacyArticlesBySection(
   return merged.slice(0, limit).map((m) => m.article);
 }
 
+// Default raised from 50 → 5000 because users expect search to return
+// every match. The 29k-article archive is small enough that title/excerpt
+// ilike filters stay fast even without an explicit cap. Pass a smaller
+// `limit` (e.g. 10) for autocomplete-style typeahead queries.
 export async function searchLegacyArticles(
   query: string,
-  limit = 50,
+  limit = 5000,
 ): Promise<Article[]> {
   const q = (query || "").trim();
   if (!q) return [];