Integrate OpenFlights API for free, no-auth flight data generation

- Added openFlightsService.js to fetch and cache OpenFlights airport/airline/routes data
- Validates airport codes exist in OpenFlights database (6072+ airports)
- Generates realistic flights using major international airlines
- Creates varied routing options: direct, 1-stop, 2-stop flights
- Updated flightService.js to use OpenFlights as primary source before Amadeus
- OpenFlights as fallback if Amadeus unavailable or returns no results
- No API keys or authentication required
- Cached locally to avoid repeated network requests
- Realistic pricing, times, and stop locations

Docker container rebuilt with OpenFlights integration.

script.js

@@ -1209,7 +1209,12 @@ function displayFlightResults(flights) {
                 <strong>${
                   flight.stops === 0
                     ? "Direct Flight"
-                    : flight.stops + " stop" + (flight.stops > 1 ? "s" : "")
+                    : flight.stops +
+                      " stop" +
+                      (flight.stops > 1 ? "s" : "") +
+                      (flight.stopCodes && flight.stopCodes.length > 0
+                        ? " (" + flight.stopCodes.join(", ") + ")"
+                        : "")
                 }</strong>
               </span>
               <span style="height: 16px; width: 1px; background: #cbd5e1;"></span>