security: add headers, fetch timeouts, Retry-After cap, env validation

- next.config.ts: CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy
- sixflags.ts: cap Retry-After at 5 min; add 15s AbortSignal.timeout()
- queuetimes.ts: add 10s AbortSignal.timeout()
- rcdb.ts: add 15s AbortSignal.timeout()
- lib/env.ts: parseStalenessHours() guards against NaN from invalid env vars
- db.ts + park-meta.ts: use parseStalenessHours() for staleness window config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/scrapers/queuetimes.ts

@@ -77,6 +77,7 @@ export async function fetchLiveRides(
     const res = await fetch(url, {
       headers: HEADERS,
       next: { revalidate },
+      signal: AbortSignal.timeout(10_000),
     } as RequestInit & { next: { revalidate: number } });
 
     if (!res.ok) return null;