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/lib/auth.ts

@@ -6,6 +6,7 @@ import { LaravelEncrypter } from "@/lib/auth/laravel-encrypter";
 import { checkLogin } from "@/lib/auth/password";
 import { verifyTotp } from "@/lib/auth/totp";
 import { prisma } from "@/lib/prisma";
+import { clientIp, rateLimit } from "@/lib/rate-limit";
 import { env } from "@/env";
 
 export const { handlers, signIn, signOut, auth } = NextAuth({
@@ -24,6 +25,9 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
         const password = String(credentials?.password ?? "");
         if (!username || !password) return null;
 
+        // Throttle login attempts per IP (10 per 5 min) against credential stuffing.
+        if (!rateLimit(`login:${await clientIp()}`, 10, 5 * 60_000).ok) return null;
+
         const user = await prisma.user.findUnique({ where: { username } });
         if (!user) return null;