docs: migration headers keep the before and after, without ids, quotes or one library's numbers (#6277)

Applies rule 32's migration exception: each header still says what the
schema was, what changes and what it prevents, but task, milestone,
decision and rule ids are replaced by the practice they stood for, the
operator quotes by the decision, single-install counts are dropped, and
pointers to a deleted package and an external design doc are gone.
golang-migrate keeps no checksum, so editing applied migrations'
comments is safe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/migrations/0048_play_events_device_class.up.sql

@@ -1,9 +1,9 @@
--- 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).
+-- 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;