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/db.ts

@@ -167,8 +167,8 @@ export function getMonthCalendar(
   return result;
 }
 
-const STALE_AFTER_MS =
-  parseInt(process.env.PARK_HOURS_STALENESS_HOURS ?? "72", 10) * 60 * 60 * 1000;
+import { parseStalenessHours } from "./env";
+const STALE_AFTER_MS = parseStalenessHours(process.env.PARK_HOURS_STALENESS_HOURS, 72) * 60 * 60 * 1000;
 
 /**
  * Returns true when the scraper should skip this park+month.