Begin tracking a booked flight

Prefiles a PIREP for the chosen bid and returns a tracking_id (the new PIREP's UUID) that subsequent /flights/update and /flights/complete calls reference.

If bid_id is omitted, the phpVMS reference implementation falls back to the pilot's most recent bid — this exists for older client builds that send an empty body. New integrations should always send bid_id.

Aircraft selection priority on the server:

1. SimBrief-attached aircraft (if the pilot has a SimBrief OFP for this flight)
2. The bid's aircraft_id (if the bid was made against a specific airframe)
3. The first available aircraft from the flight's subfleets

If none can be resolved, the server returns 500 with "No aircraft attached to bid".