Design a flight search platform

Design a large-scale flight search service similar to a consumer travel metasearch product. Users should be able to search one-way and round-trip itineraries across many airlines, filter by departure window, number of stops, airline, cabin, and price, sort by cheapest or fastest, and browse nearby dates in a price calendar. The system should return results with low latency while keeping fares and seat availability reasonably fresh even though upstream airline data changes frequently. Discuss the APIs, core data model, ingestion pipeline from airline or GDS feeds, indexing and search strategy, caching, ranking, and how you would handle stale prices, updates, and scaling.