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/playevents/writer.go

@@ -65,7 +65,16 @@ func (w *Writer) RecordPlayStarted(
 	clientID string,
 	at time.Time,
 ) (StartedResult, error) {
-	return w.RecordPlayStartedWithSource(ctx, userID, trackID, clientID, "", at)
+	return w.RecordPlayStartedWithSource(ctx, userID, trackID, clientID, "", "", at)
+}
+
+// strPtrOrNil maps an empty string to a NULL text column (nullable pointer),
+// mirroring the clientID/source nil-on-empty pattern used above.
+func strPtrOrNil(s string) *string {
+	if s == "" {
+		return nil
+	}
+	return &s
 }
 
 // systemPlaylistSources are the play_events.source values that count
@@ -136,7 +145,7 @@ func lookupSystemPickKind(
 func (w *Writer) RecordPlayStartedWithSource(
 	ctx context.Context,
 	userID, trackID pgtype.UUID,
-	clientID, source string,
+	clientID, source, deviceClass string,
 	at time.Time,
 ) (StartedResult, error) {
 	var out StartedResult
@@ -165,13 +174,14 @@ func (w *Writer) RecordPlayStartedWithSource(
 			}
 		}
 		ev, err := q.InsertPlayEvent(ctx, dbq.InsertPlayEventParams{
-			UserID:    userID,
-			TrackID:   trackID,
-			SessionID: sessionID,
-			StartedAt: pgtype.Timestamptz{Time: at, Valid: true},
-			ClientID:  clientIDPtr,
-			Source:    sourcePtr,
-			PickKind:  pickKind,
+			UserID:      userID,
+			TrackID:     trackID,
+			SessionID:   sessionID,
+			StartedAt:   pgtype.Timestamptz{Time: at, Valid: true},
+			ClientID:    clientIDPtr,
+			Source:      sourcePtr,
+			PickKind:    pickKind,
+			DeviceClass: strPtrOrNil(deviceClass),
 		})
 		if err != nil {
 			return err
@@ -389,7 +399,7 @@ func (w *Writer) RecordSyntheticCompletedPlay(
 func (w *Writer) RecordOfflinePlay(
 	ctx context.Context,
 	userID, trackID pgtype.UUID,
-	clientID, source string,
+	clientID, source, deviceClass string,
 	at time.Time,
 	durationPlayedMs int32,
 ) error {
@@ -441,13 +451,14 @@ func (w *Writer) RecordOfflinePlay(
 			}
 		}
 		ev, err := q.InsertPlayEvent(ctx, dbq.InsertPlayEventParams{
-			UserID:    userID,
-			TrackID:   trackID,
-			SessionID: sessionID,
-			StartedAt: pgtype.Timestamptz{Time: at, Valid: true},
-			ClientID:  clientIDPtr,
-			Source:    sourcePtr,
-			PickKind:  pickKind,
+			UserID:      userID,
+			TrackID:     trackID,
+			SessionID:   sessionID,
+			StartedAt:   pgtype.Timestamptz{Time: at, Valid: true},
+			ClientID:    clientIDPtr,
+			Source:      sourcePtr,
+			PickKind:    pickKind,
+			DeviceClass: strPtrOrNil(deviceClass),
 		})
 		if err != nil {
 			return err