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_test.go

@@ -226,10 +226,10 @@ func TestRecordOfflinePlay_DedupsByUserTrackStartedAt(t *testing.T) {
 	f := newFixture(t, 200_000)
 	ctx := context.Background()
 	at := time.Now().UTC()
-	if err := f.w.RecordOfflinePlay(ctx, f.user, f.track, "c", "", at, 120_000); err != nil {
+	if err := f.w.RecordOfflinePlay(ctx, f.user, f.track, "c", "", "", at, 120_000); err != nil {
 		t.Fatalf("first offline: %v", err)
 	}
-	if err := f.w.RecordOfflinePlay(ctx, f.user, f.track, "c", "", at, 120_000); err != nil {
+	if err := f.w.RecordOfflinePlay(ctx, f.user, f.track, "c", "", "", at, 120_000); err != nil {
 		t.Fatalf("replay offline: %v", err)
 	}
 	var count int
@@ -444,7 +444,7 @@ func TestRecordPlayStartedWithSource_AppendsRotation(t *testing.T) {
 	f := newFixture(t, 200_000)
 	now := time.Now().UTC()
 	if _, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "for_you", now,
+		context.Background(), f.user, f.track, "c", "for_you", "", now,
 	); err != nil {
 		t.Fatalf("RecordPlayStartedWithSource: %v", err)
 	}
@@ -460,7 +460,7 @@ func TestRecordPlayStartedWithSource_AppendsRotation(t *testing.T) {
 
 	// Re-playing the same track stays a set (no duplicate append).
 	if _, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "for_you", now.Add(time.Minute),
+		context.Background(), f.user, f.track, "c", "for_you", "", now.Add(time.Minute),
 	); err != nil {
 		t.Fatalf("RecordPlayStartedWithSource 2: %v", err)
 	}
@@ -511,7 +511,7 @@ func TestRecordPlayStartedWithSource_StampsForYouPickKind(t *testing.T) {
 	f := newFixture(t, 200_000)
 	seedSystemSnapshot(t, f, "for_you", "fresh")
 	res, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "for_you", time.Now().UTC(),
+		context.Background(), f.user, f.track, "c", "for_you", "", time.Now().UTC(),
 	)
 	if err != nil {
 		t.Fatalf("RecordPlayStartedWithSource: %v", err)
@@ -530,7 +530,7 @@ func TestRecordPlayStartedWithSource_NoSnapshotMatch_PickKindNull(t *testing.T)
 	// out, or no snapshot exists) stays unattributed rather than guessing.
 	f := newFixture(t, 200_000)
 	res, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "for_you", time.Now().UTC(),
+		context.Background(), f.user, f.track, "c", "for_you", "", time.Now().UTC(),
 	)
 	if err != nil {
 		t.Fatalf("RecordPlayStartedWithSource: %v", err)
@@ -552,7 +552,7 @@ func TestRecordPlayStartedWithSource_CrossVariant_PickKindNull(t *testing.T) {
 	f := newFixture(t, 200_000)
 	seedSystemSnapshot(t, f, "for_you", "taste")
 	res, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "discover", time.Now().UTC(),
+		context.Background(), f.user, f.track, "c", "discover", "", time.Now().UTC(),
 	)
 	if err != nil {
 		t.Fatalf("RecordPlayStartedWithSource: %v", err)
@@ -572,7 +572,7 @@ func TestRecordPlayStartedWithSource_StampsDiscoverBucket(t *testing.T) {
 	f := newFixture(t, 200_000)
 	seedSystemSnapshot(t, f, "discover", "dormant")
 	res, err := f.w.RecordPlayStartedWithSource(
-		context.Background(), f.user, f.track, "c", "discover", time.Now().UTC(),
+		context.Background(), f.user, f.track, "c", "discover", "", time.Now().UTC(),
 	)
 	if err != nil {
 		t.Fatalf("RecordPlayStartedWithSource: %v", err)
@@ -593,7 +593,7 @@ func TestRecordOfflinePlay_StampsForYouPickKind(t *testing.T) {
 	seedSystemSnapshot(t, f, "for_you", "taste")
 	at := time.Now().UTC().Add(-time.Hour)
 	if err := f.w.RecordOfflinePlay(
-		context.Background(), f.user, f.track, "c", "for_you", at, 180_000,
+		context.Background(), f.user, f.track, "c", "for_you", "", at, 180_000,
 	); err != nil {
 		t.Fatalf("RecordOfflinePlay: %v", err)
 	}