-- 0048_play_events_device_class.up.sql — device-class context conditioning.
-- Records which class of device a play came from (mobile / web / …), sent by
-- the client on play-start and normalized server-side. Extends the
-- time-of-day/weekday affinity cell (0046) to (daypart × weekday × device) for
-- the radio path. Nullable + no CHECK: values are normalized in Go, so a new
-- client class needs no migration, and historical rows stay NULL → excluded
-- from the device dimension (a graceful cold-start; the facet ramps as new
-- plays land).
ALTER TABLE play_events ADD COLUMN device_class text;