-- 0048_play_events_device_class.up.sql — device-class context conditioning
-- (#1551, milestone #160 Opt 3b). Records which class of device a play came
-- from (mobile / web / …), sent by the client on play-start and normalized
-- server-side. Extends the #1531 time-of-day/weekday affinity cell to
-- (daypart × weekday × device) for the radio path. Nullable + no CHECK: values
-- are normalized in Go (avoids a CHECK-migration each time a client class is
-- added, rule #36), 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;