- GET /api/directions proxies OSRM (OSRM_URL env, default public demo
server) with validation, Leaflet-order geometry, 24h capped cache
- Map draws straight lines first, then upgrades ground legs to
road-following polylines with routed km (marked road) as directions
arrive; silent fallback to great-circle on failure; air legs unchanged
- README documents OSRM_URL