Return a GeoJSON FeatureCollection of Shinkansen lines.

Each feature's geometry is a MultiLineString in WGS84 (the underlying PostGIS column is geometry(MultiLineString, 4326)). Per-feature properties carry the names + operator labels the FE needs to label each line. Brand COLORS live on the FE side as a static map (see program/app/japan/rail-line-colors.ts) — they're presentation, not data, and don't need a DB round-trip.

Cache headers match japan_places: short browser TTL, longer at the edge — data only changes when the loader script runs.