editoast: implement /level_crossing_occupancy endpoint logic

- Load infra, level crossings and trains
- Run train pathfinding and simulation
- Compute occupancy time windows
- Group results by level crossing ID
- Skip trains with missing rolling stock, failed simulations, no pedal intersection, or that do not fully traverse the short zone
- Add i18n messages for level_crossing errors in en and fr

Signed-off-by: aschantraine <[email protected]>

Author: aschantraine