perf(landing): WebP welcome screenshots + long-cache static assets

The 3 landing screenshots were 2.3 MB of PNG (feed.png alone 1.6 MB, decoded
at 2400x1350 for a ~400px slot) — the biggest LCP/transfer cost on the public
page. Re-encoded to WebP capped at 1600px (~284 KB total, feed 91% smaller).
Also set Cache-Control: content-hashed /assets/* are immutable for a year,
other SPA-root static files (welcome images, favicon, robots) get a 7-day TTL;
index.html stays no-cache. Register image/webp+avif mimetypes so FileResponse
serves the right Content-Type. Lighthouse (dev): Perf 80->93, LCP 1.8s->1.2s.

frontend/src/components/Welcome.tsx

@@ -59,7 +59,7 @@ export default function Welcome() {
 
         {/* App preview */}
         <Preview
-          src="/welcome/feed.png"
+          src="/welcome/feed.webp"
           label={t("welcome.preview.feed")}
           className="aspect-video"
           onOpen={open}
@@ -101,13 +101,13 @@ export default function Welcome() {
         {/* Secondary showcase — smaller thumbnails; click to view full size. */}
         <section className="mt-12 flex flex-wrap justify-center gap-4">
           <Preview
-            src="/welcome/channels.png"
+            src="/welcome/channels.webp"
             label={t("welcome.preview.channels")}
             className="w-full sm:w-72 aspect-[4/3]"
             onOpen={open}
           />
           <Preview
-            src="/welcome/playlists.png"
+            src="/welcome/playlists.webp"
             label={t("welcome.preview.playlists")}
             className="w-full sm:w-72 aspect-[4/3]"
             onOpen={open}