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/next.config.js

@@ -1,5 +1,9 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  // Isolate dev and prod build output. `next start` (production) reads the
+  // default `.next`; `next dev` is pointed at `.next-dev` via NEXT_DIST_DIR so a
+  // stray dev run can never overwrite the live production manifests.
+  distDir: process.env.NEXT_DIST_DIR || '.next',
   experimental: {
     serverComponentsExternalPackages: ['sharp'],
   },