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/Classes.tsx

@@ -53,6 +53,8 @@ export function Classes({ data: classes }: ClassesProps) {
                         src={item.images[0]}
                         alt={item.name}
                         fill
+                        loading="lazy"
+                        sizes="(min-width: 1024px) 60vw, 100vw"
                         className="object-cover"
                       />
                       {/* Gradient overlay */}