feat: Implement synthetic edges city↔airport (Task 10)

- Add TransferTime constants (AirportToCity, CityToStation, StationToStation)
- Update Edge struct with Synthetic field
- Enhance addSyntheticEdgesForNode to use constants and mark synthetic edges
- Update BuildGraphFromStations to set Synthetic field
- Mark synthetic edges in GeoJSON output as dashed lines
- Write TestSyntheticAirportCityEdges and TestRouteWithSyntheticAirportCityEdges tests

docs/plans/2026-08-15-full-implementation.md

@@ -117,12 +117,12 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] **Write tests:** TestTransportTypesInGraph
 - [x] Run tests - must pass before task 10
 
-### Task 10: Synthetic edges "город↔аэропорт" [ ]
-- [ ] Implement synthetic edges for airport-city transfers
-- [ ] Add constants for transfer time estimation (section 7.4)
-- [ ] Mark synthetic edges in GeoJSON output (dashed line)
-- [ ] **Write tests:** TestSyntheticAirportCityEdges
-- [ ] Run tests - must pass before task 11
+### Task 10: Synthetic edges "город↔аэропорт" [x]
+- [x] Implement synthetic edges for airport-city transfers
+- [x] Add constants for transfer time estimation (section 7.4)
+- [x] Mark synthetic edges in GeoJSON output (dashed line)
+- [x] **Write tests:** TestSyntheticAirportCityEdges
+- [x] Run tests - must pass before task 11
 
 ### Task 11: MCT rules implementation [ ]
 - [ ] Create `transfer_rules` table migration