perf: reduce web requests with caching, bot control, and deferred JS

Add robots.txt to block aggressive AI scrapers (GPTBot, Bytespider,
SemrushBot, etc.) and set crawl delays for legitimate search engines.

Add Cache-Control headers for HTML pages (1h TTL with
stale-while-revalidate) so repeat visits are served from Netlify's
CDN edge cache instead of origin.

Defer below-fold video components from client:load to client:visible
so their JS chunks only load when scrolled into view — bots and users
who don't scroll never trigger those requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/_headers

@@ -0,0 +1,25 @@
+# HTML pages — 1 hour cache, serve stale while revalidating
+/*
+  Cache-Control: public, max-age=3600, stale-while-revalidate=86400
+
+# Hashed build assets — immutable forever
+/_astro/*
+  Cache-Control: public, max-age=31536000, immutable
+
+/fonts/*
+  Cache-Control: public, max-age=31536000, immutable
+
+/favicon.ico
+  Cache-Control: public, max-age=604800
+
+/favicon.svg
+  Cache-Control: public, max-age=604800
+
+/*.svg
+  Cache-Control: public, max-age=86400
+
+/*.pdf
+  Cache-Control: public, max-age=604800
+
+/sitemap-*.xml
+  Cache-Control: public, max-age=3600

public/robots.txt

@@ -0,0 +1,66 @@
+# Audacity Website - robots.txt
+
+# Allow legitimate search engines with a crawl delay
+User-agent: Googlebot
+Crawl-delay: 2
+
+User-agent: Bingbot
+Crawl-delay: 2
+
+User-agent: DuckDuckBot
+Crawl-delay: 5
+
+User-agent: Yandex
+Crawl-delay: 10
+
+# Block AI scrapers and aggressive bots
+User-agent: GPTBot
+Disallow: /
+
+User-agent: ChatGPT-User
+Disallow: /
+
+User-agent: Google-Extended
+Disallow: /
+
+User-agent: CCBot
+Disallow: /
+
+User-agent: anthropic-ai
+Disallow: /
+
+User-agent: Claude-Web
+Disallow: /
+
+User-agent: Bytespider
+Disallow: /
+
+User-agent: PetalBot
+Disallow: /
+
+User-agent: Sogou
+Disallow: /
+
+User-agent: SemrushBot
+Disallow: /
+
+User-agent: AhrefsBot
+Disallow: /
+
+User-agent: MJ12bot
+Disallow: /
+
+User-agent: DotBot
+Disallow: /
+
+User-agent: BLEXBot
+Disallow: /
+
+User-agent: DataForSeoBot
+Disallow: /
+
+# Default: allow everything, but slow down
+User-agent: *
+Crawl-delay: 5
+
+Sitemap: https://www.audacityteam.org/sitemap-index.xml

src/components/homepage/ReleaseVideo.astro

@@ -13,11 +13,11 @@ const videoPromos = getFilteredPromos(Object.values(promoData), {
   >
     <div class="flex flex-col md:flex-row gap-12 md:gap-16">
       <div class="w-full md:w-1/2">
-        <SplitFeaturedVideo client:load promos={videoPromos} slot="1" />
+        <SplitFeaturedVideo client:visible promos={videoPromos} slot="1" />
       </div>
 
       <div class="w-full md:w-1/2">
-        <SplitFeaturedVideo client:load promos={videoPromos} slot="2" />
+        <SplitFeaturedVideo client:visible promos={videoPromos} slot="2" />
       </div>
     </div>
   </div>