BUG: Address broken NEXRAD radar when key expires

[cwdaniel]

● Summary

• Fix NEXRAD station data permanently disappearing from Redis after key expiry
• Root cause: redis.expire() on a non-existent key silently returns 0, so the "data unchanged" path kept refreshing dead keys and skipping re-downloads indefinitely

Changes

• src/nexrad/redis-scan-store.ts: refreshScanTTL now returns boolean — true if the scan key still exists, false if it has expired
• src/nexrad/main.ts: When refreshScanTTL returns false, clear latestVolume for that station to force a re-download on the same poll cycle. Stations self-heal within 60
  seconds instead of staying permanently unavailable

Testing

How did you verify this works?

• npx tsc --noEmit passes
• Docker builds successfully
• Tested locally with docker compose up
• Verified in browser at radrview.com
• Confirmed docker exec radrview-redis redis-cli keys 'nexrad:status:*' returns empty before fix
• Confirmed stations recover after docker restart radrview-ingest-nexrad