fix: critical perf & security — rate limiting, DB indexes, N+1 query, image lazy loading

- Add in-memory rate limiter (src/lib/rateLimit.ts) to public registration endpoints
- Add DB migration #9 with 8 performance indexes on booking/registration tables
- Fix N+1 query in getUpcomingReminders() — single IN() query instead of per-title
- Add loading="lazy" to all non-hero images (MasterClasses, News, Classes, Team)
- Add sizes attribute to Classes images for better responsive loading

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/sections/MasterClasses.tsx

@@ -98,6 +98,7 @@ function MasterClassCard({
             src={item.image}
             alt={item.title}
             fill
+            loading="lazy"
             sizes="(min-width: 1024px) 33vw, (min-width: 640px) 50vw, 100vw"
             className="object-cover transition-transform duration-700 group-hover:scale-110"
           />