feat(taste): device-class context conditioning — #1551

Milestone #160 Opt 3b. Adds device class as a third context axis on top
of the #1531 time-of-day/weekday affinity: on the radio path, a candidate
is boosted when its artist concentrates in the current (daypart × weekday
× device) cell. Client-sent (client_id is opaque; no UA stored), so it's
captured going forward and applies to radio only (daily mixes are
cron-built with no device → stay device-agnostic).

Server:
- Migration 0048: play_events.device_class text NULL (no CHECK; normalized
  in Go — one whitelist entry per new client class, not a migration).
- events.go: eventRequest.device_class + normalizeDeviceClass (whitelist →
  mobile/web/…, else "other", empty → NULL); threaded through both
  RecordPlayStartedWithSource and RecordOfflinePlay into InsertPlayEvent.
- ListArtistContextPlayCountsForUser gains a current-device param; the cell
  FILTER adds AND ($2='' OR device_class=$2) — '' reproduces the #1531
  time-only behaviour exactly (used by mixes). SessionVector.DeviceClass
  carries it; the radio handler derives the current device from the user's
  latest play (GetLatestPlayDeviceClassForUser) — request-free proxy.
- No new tuning knob: device narrows the existing ContextAffinityScore
  (reuses context_time_weight).

Clients:
- web: play_started sends device_class 'web'.
- android: play_started + offline replay send 'mobile' (EventsWire +
  PlayOfflinePayload + MutationReplayer + PlayEventsReporter).

Test: LoadContextAffinity device-narrowing integration test (mobile vs web
artist separation; device-agnostic parity).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/db/dbq/recommendation.sql.go

@@ -11,6 +11,24 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const getLatestPlayDeviceClassForUser = `-- name: GetLatestPlayDeviceClassForUser :one
+SELECT device_class
+FROM play_events
+WHERE user_id = $1
+ORDER BY started_at DESC
+LIMIT 1
+`
+
+// The device_class of the user's most recent play (#1551), used as the
+// "current device" for radio context conditioning. NULL when the latest play
+// predates device capture or came from a client that didn't send one.
+func (q *Queries) GetLatestPlayDeviceClassForUser(ctx context.Context, userID pgtype.UUID) (*string, error) {
+	row := q.db.QueryRow(ctx, getLatestPlayDeviceClassForUser, userID)
+	var device_class *string
+	err := row.Scan(&device_class)
+	return device_class, err
+}
+
 const listArtistContextPlayCountsForUser = `-- name: ListArtistContextPlayCountsForUser :many
 WITH tz AS (
     SELECT COALESCE(NULLIF(u.timezone, ''), 'UTC') AS zone
@@ -30,6 +48,7 @@ now_cell AS (
 ),
 plays AS (
     SELECT t.artist_id,
+           pe.device_class,
            CASE
                WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 5  THEN 3
                WHEN EXTRACT(hour FROM pe.started_at AT TIME ZONE tz.zone) < 12 THEN 0
@@ -50,11 +69,17 @@ SELECT p.artist_id,
        count(*) FILTER (
            WHERE p.daypart = (SELECT daypart FROM now_cell)
              AND p.is_weekend = (SELECT is_weekend FROM now_cell)
+             AND ($2::text = '' OR p.device_class = $2::text)
        ) AS cell_plays
 FROM plays p
 GROUP BY p.artist_id
 `
 
+type ListArtistContextPlayCountsForUserParams struct {
+	ID      pgtype.UUID
+	Column2 string
+}
+
 type ListArtistContextPlayCountsForUserRow struct {
 	ArtistID   pgtype.UUID
 	TotalPlays int64
@@ -62,14 +87,16 @@ type ListArtistContextPlayCountsForUserRow struct {
 }
 
 // Per-artist completed-play counts split by whether each play falls in the
-// CURRENT daypart × weekday-type cell, in the user's local timezone (#1531).
-// Feeds the context-affinity scoring term: an artist whose plays concentrate
-// in the current cell (vs the user's overall baseline, computed Go-side from
-// these rows) gets boosted right now. Skips excluded; a 365-day window bounds
-// cost. Daypart buckets: night [22,5) morning [5,12) afternoon [12,17)
-// evening [17,22). Weekend = ISO days 6–7 (Sat/Sun).
-func (q *Queries) ListArtistContextPlayCountsForUser(ctx context.Context, id pgtype.UUID) ([]ListArtistContextPlayCountsForUserRow, error) {
-	rows, err := q.db.Query(ctx, listArtistContextPlayCountsForUser, id)
+// CURRENT context cell, in the user's local timezone (#1531). The cell is
+// daypart × weekday-type, optionally narrowed by device class (#1551): when
+// $2 (the current device) is ”, the device dimension is ignored (identical to
+// the time-only #1531 behaviour, used by the daily mixes which have no device);
+// when $2 is set (radio), a play only counts in the cell if its device_class
+// matches. Feeds the context-affinity scoring term. Skips excluded; a 365-day
+// window bounds cost. Daypart buckets: night [22,5) morning [5,12)
+// afternoon [12,17) evening [17,22). Weekend = ISO days 6–7 (Sat/Sun).
+func (q *Queries) ListArtistContextPlayCountsForUser(ctx context.Context, arg ListArtistContextPlayCountsForUserParams) ([]ListArtistContextPlayCountsForUserRow, error) {
+	rows, err := q.db.Query(ctx, listArtistContextPlayCountsForUser, arg.ID, arg.Column2)
 	if err != nil {
 		return nil, err
 	}