refactor: production-essentials hardening pass

Backend: structured logger, env-validated config, graceful SIGTERM/SIGINT
shutdown, per-IP rate limiter, per-tier scheduler concurrency latch, error
context on previously-silent catches, compiled-JS Dockerfile stage.

Frontend: lib/api.ts consolidates BACKEND_URL with lazy production-required
check, root + per-segment error.tsx / not-found.tsx / loading.tsx,
generateMetadata on park and ride pages, graceful fallback when backend is
unreachable, Plausible script gated on env vars.

Infra: CI runs lint + typecheck + tests on both packages before docker build,
compose adds healthchecks, log rotation, and memory limits; .env.example
documents every variable.

Cleanup: removed empty app/api/parks/ dir and 0-byte root parks.db, moved
wait-times-urls.txt into docs/, dropped an `as any` cast.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/not-found.tsx

@@ -0,0 +1,31 @@
+import Link from "next/link";
+
+export default function NotFound() {
+  return (
+    <main style={{ minHeight: "100vh", display: "grid", placeItems: "center", padding: 24, background: "var(--color-bg)" }}>
+      <div style={{ maxWidth: 480, textAlign: "center" }}>
+        <div style={{ fontSize: "2.5rem", fontWeight: 700, color: "var(--color-text)", marginBottom: 8 }}>404</div>
+        <h1 style={{ fontSize: "1.1rem", fontWeight: 600, color: "var(--color-text)", marginBottom: 12 }}>
+          Page not found
+        </h1>
+        <p style={{ color: "var(--color-text-muted)", lineHeight: 1.6, fontSize: "0.9rem", marginBottom: 20 }}>
+          We couldn&apos;t find what you were looking for.
+        </p>
+        <Link
+          href="/"
+          style={{
+            padding: "8px 18px",
+            background: "var(--color-text)",
+            color: "var(--color-bg)",
+            borderRadius: 6,
+            fontSize: "0.85rem",
+            fontWeight: 600,
+            textDecoration: "none",
+          }}
+        >
+          Back to the calendar
+        </Link>
+      </div>
+    </main>
+  );
+}