Production hardening: error pages, rate limiting, metadata

- Custom not-found (404) + error / global-error boundaries, styled with
  the public design system; raw errors logged, never shown to users.
- In-process rate limiter (src/lib/rate-limit.ts) wired into the abuse-
  prone flows: login (10/5min/IP), register (5/10min/IP), password-reset
  request (3/15min/IP), keyed by the proxy-forwarded client IP.
- SEO/metadata: root generateMetadata sets a `%s · {hotel}` title
  template from the live hotel_name; dynamic generateMetadata on
  news/[slug] (article title + excerpt) and u/[username] (name + motto);
  static titles on 12 primary public pages.
- env.ts: added the vars introduced since (PASSWORD_HASH, OPENAI_API_KEY,
  DISCORD_WEBHOOK_URL, ALERT_EMAIL, PAYPAL_*) so env stays authoritative.

Verified on the prod server: /missing → 404 card, news title renders
"News · Habbo". tsc 0, vitest 49/49, next build 0.

src/app/leaderboard/page.tsx

@@ -6,6 +6,8 @@ import { siteSettings } from "@/lib/services/site-settings";
 
 export const dynamic = "force-dynamic";
 
+export const metadata = { title: "Leaderboard" };
+
 // AtomCMS-faithful currency type ids (see prisma/schema.prisma UsersCurrency):
 //   Credits = -1 (lives on users.credits), Duckets = 0, Diamonds = 5.
 const TABS = [