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>

.gitignore

@@ -3,6 +3,9 @@ node_modules/
 
 # Next.js
 frontend/.next/
+frontend/.next-dev/
+frontend/.next-build/
+frontend/.next-prev/
 frontend/out/
 
 # Backend build