fix: harden deploys and close top security holes after /events outage

Isolate next dev from production .next, add build-guard/atomic deploy/health
watchdog, error boundaries, and fix JWT startup, meetup leaks, media path
traversal, SVG/memory uploads, and JSON-LD escaping.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/public/JsonLd.tsx

@@ -3,10 +3,13 @@ interface JsonLdProps {
 }
 
 export function JsonLd({ data }: JsonLdProps) {
+  // Escape `<` so user-controlled titles/descriptions cannot break out of the
+  // <script> tag via `</script>` (JSON.stringify alone does not escape it).
+  const json = JSON.stringify(data).replace(/</g, "\\u003c");
   return (
     <script
       type="application/ld+json"
-      dangerouslySetInnerHTML={{ __html: JSON.stringify(data) }}
+      dangerouslySetInnerHTML={{ __html: json }}
     />
   );
 }