SSTV mode was slow to populate next-pass countdown and ISS location map
due to uncached skyfield computation and sequential JS API calls.
- Cache ISS position (10s TTL) and schedule (15min TTL, keyed by rounded lat/lon)
- Cache skyfield timescale object (expensive to create on every request)
- Reduce external API timeouts from 5s to 3s
- Fire checkStatus, loadImages, loadIssSchedule, updateIssPosition in parallel via Promise.all
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>