Re-enable live flight tracking

[JEK58]

Summary

• restore the authenticated /api/live endpoint and homepage live-flight data
• restart scheduled FLARM collection between 08:00 and 21:59
• add overlap protection, request timeouts, and filtering for empty or invalid FLARM IDs
• add LIVE_TRACKING_ENABLED so collection can be disabled through server configuration without changing code
• disable live tracking in CI and document the FLARM settings in .env-sample

Why

Live tracking had been commented out after earlier server errors, even though the client UI remained in place. The upstream FLARM endpoint is responding successfully again, so this restores the integration while adding a configuration switch and safeguards that make it safer to operate and disable.

Impact

Authenticated users can see live flights on the live map and homepage again when LIVE_TRACKING_ENABLED=true and a valid FLARM_API_KEY is configured. Setting the flag to false prevents the collection cron job from starting after a server restart.

Validation

• ./node_modules/.bin/jest test/LivetrackingService.test.ts --runInBand
• 2 focused tests passed, covering enabled collection/scoring/caching and disabled scheduling
• git diff --check