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>

backend/src/api/calendar.ts

@@ -91,7 +91,12 @@ router.get('/ics', async (_req: Request, res: Response) => {
     const cutoff = sevenDaysAgo.toISOString().slice(0, 10);
 
     const meetups = await prisma.meetup.findMany({
-      where: { date: { gte: cutoff } },
+      // Public subscription feed — never expose HIDDEN or unpublished meetups.
+      where: {
+        date: { gte: cutoff },
+        visibility: 'PUBLIC',
+        status: 'PUBLISHED',
+      },
       orderBy: { date: 'asc' },
       include: { organizer: true },
     });