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/u/[username]/page.tsx

@@ -1,3 +1,4 @@
+import type { Metadata } from "next";
 import { notFound } from "next/navigation";
 import { ContentCard, EmptyState, OnlineBadge, StatBlock } from "@/components/public/ui";
 import { auth } from "@/lib/auth";
@@ -8,6 +9,19 @@ import { postGuestbook } from "@/actions/guestbook";
 
 export const dynamic = "force-dynamic";
 
+export async function generateMetadata({
+  params,
+}: {
+  params: Promise<{ username: string }>;
+}): Promise<Metadata> {
+  const { username } = await params;
+  const user = await prisma.user
+    .findUnique({ where: { username }, select: { username: true, motto: true } })
+    .catch(() => null);
+  if (!user) return { title: "Profile" };
+  return { title: user.username, description: user.motto || `${user.username}'s profile` };
+}
+
 // Canonical Habbo badge image CDN. Badge codes (e.g. "ADM") map to a .gif here.
 const BADGE_IMG_BASE = "https://images.habbo.com/c_images/album1584";