Ski: an area's run list now includes its backcountry-tagged runs

GET /ski/areas/{id}/features left out runs that OpenStreetMap treats as backcountry, such as most double blacks, even though the ski tiles draw them. Release 2026-09-17.1fbcd352ff00 lists them: 1,452 more runs across 115 areas.

• A run tagged piste:difficulty expert, freeride or extreme with no piste:grooming, or tagged piste:grooming=backcountry, is drawn by the ski tiles but was often missing from its area's /features list, so an app built on the list could not show or set it. Pass Powderkeg was missing all four of its double blacks.
• Such a run now belongs to the nearest area whose runs or lifts are within 500 m of it.
• Nothing else changed: every run and lift already listed keeps its id and its data, the tiles are the same, and the /ski/areas records are unchanged.