Cache reads now accept an optional ttl (seconds). get_cached checks the
file mtime and returns None if the entry is older than the TTL, triggering
a fresh fetch. Eurostar prices use a 24-hour TTL; timetable caches remain
indefinite (date-scoped keys become irrelevant once the date passes).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
