Ski: stable ids for every run and lift

GET /ski/areas/{id}/features lists an area's runs and lifts, one per OpenStreetMap element, each with an id that stays the same from one release to the next. The ski tiles carry the same ids.

• GET /ski/areas/{id}/features returns every run and lift in the area with id, kind, name, piste_type, difficulty, aerialway, GeoJSON geometry, length_m and an osm reference.
• An id is way-<n> or relation-<n>: the OpenStreetMap element. It does not change while the element exists. A lift's id is the one /lifts-3d already gives it.
• The runs and lifts features of the ski tiles carry the same id, and the style sets promoteId: "id", so a map can colour a run by your own data with feature-state.

AvailabilityBehind the Ski add-on, like the other /ski endpoints. Live once the release is published and SKI_RELEASE names it.