perf: add ISR to /prompts page (60s revalidation)

This page has no auth() dependency and gets high traffic with
0% cache hit rate. ISR with 60s revalidation serves most
requests from cache while keeping content fresh.

src/app/prompts/page.tsx

@@ -16,6 +16,8 @@ import { isAISearchEnabled, semanticSearch } from "@/lib/ai/embeddings";
 import { isAIGenerationEnabled } from "@/lib/ai/generation";
 import config from "@/../prompts.config";
 
+export const revalidate = 60;
+
 export const metadata: Metadata = {
   title: "Prompts",
   description: "Browse and discover AI prompts",