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/events.sql.go

@@ -54,7 +54,7 @@ func (q *Queries) GetMostRecentPlaySessionForUser(ctx context.Context, userID pg
 }
 
 const getOpenPlayEventForUser = `-- name: GetOpenPlayEventForUser :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind FROM play_events
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind, device_class FROM play_events
 WHERE user_id = $1 AND ended_at IS NULL
 ORDER BY started_at DESC
 LIMIT 1
@@ -80,12 +80,13 @@ func (q *Queries) GetOpenPlayEventForUser(ctx context.Context, userID pgtype.UUI
 		&i.ScrobbledAt,
 		&i.Source,
 		&i.PickKind,
+		&i.DeviceClass,
 	)
 	return i, err
 }
 
 const getPlayEventByID = `-- name: GetPlayEventByID :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind FROM play_events WHERE id = $1
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind, device_class FROM play_events WHERE id = $1
 `
 
 func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEvent, error) {
@@ -106,6 +107,7 @@ func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEve
 		&i.ScrobbledAt,
 		&i.Source,
 		&i.PickKind,
+		&i.DeviceClass,
 	)
 	return i, err
 }
@@ -144,19 +146,22 @@ func (q *Queries) GetSystemPickKindForTrack(ctx context.Context, arg GetSystemPi
 
 const insertPlayEvent = `-- name: InsertPlayEvent :one
 INSERT INTO play_events (
-    user_id, track_id, session_id, started_at, client_id, source, pick_kind
-) VALUES ($1, $2, $3, $4, $5, $6, $7::text)
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind
+    user_id, track_id, session_id, started_at, client_id, source, pick_kind,
+    device_class
+) VALUES ($1, $2, $3, $4, $5, $6, $7::text,
+    $8::text)
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind, device_class
 `
 
 type InsertPlayEventParams struct {
-	UserID    pgtype.UUID
-	TrackID   pgtype.UUID
-	SessionID pgtype.UUID
-	StartedAt pgtype.Timestamptz
-	ClientID  *string
-	Source    *string
-	PickKind  *string
+	UserID      pgtype.UUID
+	TrackID     pgtype.UUID
+	SessionID   pgtype.UUID
+	StartedAt   pgtype.Timestamptz
+	ClientID    *string
+	Source      *string
+	PickKind    *string
+	DeviceClass *string
 }
 
 // pick_kind is non-NULL only for system-playlist plays whose track was
@@ -171,6 +176,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		arg.ClientID,
 		arg.Source,
 		arg.PickKind,
+		arg.DeviceClass,
 	)
 	var i PlayEvent
 	err := row.Scan(
@@ -188,6 +194,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		&i.ScrobbledAt,
 		&i.Source,
 		&i.PickKind,
+		&i.DeviceClass,
 	)
 	return i, err
 }
@@ -333,7 +340,7 @@ SET ended_at = $2,
     completion_ratio = $4,
     was_skipped = $5
 WHERE id = $1
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source, pick_kind, device_class
 `
 
 type UpdatePlayEventEndedParams struct {
@@ -371,6 +378,7 @@ func (q *Queries) UpdatePlayEventEnded(ctx context.Context, arg UpdatePlayEventE
 		&i.ScrobbledAt,
 		&i.Source,
 		&i.PickKind,
+		&i.DeviceClass,
 	)
 	return i, err
 }