Route ground legs along real roads via an OSRM directions proxy

- 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

public/js/api.js

@@ -73,6 +73,7 @@ export const api = {
     costs: (id) => get(`/api/trips/${id}/costs`),
     regenerateTransports: (id) => post(`/api/trips/${id}/transports/regenerate`, {}),
   },
+  directions: (from, to) => get(`/api/directions?from=${from.lat},${from.lng}&to=${to.lat},${to.lng}`),
   entries: {
     create: (tripId, payload) => post(`/api/trips/${tripId}/entries`, payload),
     update: (id, patchBody) => patch(`/api/entries/${id}`, patchBody),