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/recommendation/context.go

@@ -28,15 +28,20 @@ func (c ContextAffinity) Affinity(artistID pgtype.UUID) float64 {
 }
 
 // LoadContextAffinity computes each artist's affinity for the user's CURRENT
-// daypart × weekday cell. For every artist with completed plays in the window
-// it compares the share of that artist's plays that fall in the current cell
+// context cell — daypart × weekday, narrowed by deviceClass when non-empty
+// (#1551; radio passes the current device, the daily mixes pass "" for a
+// device-agnostic cell). For every artist with completed plays in the window it
+// compares the share of that artist's plays that fall in the current cell
 // against the user's overall baseline share, shrinking sparse artists toward
 // the baseline. Returns an empty (all-neutral) affinity when the user has no
 // plays.
 func LoadContextAffinity(
-	ctx context.Context, q *dbq.Queries, userID pgtype.UUID,
+	ctx context.Context, q *dbq.Queries, userID pgtype.UUID, deviceClass string,
 ) (ContextAffinity, error) {
-	rows, err := q.ListArtistContextPlayCountsForUser(ctx, userID)
+	rows, err := q.ListArtistContextPlayCountsForUser(ctx, dbq.ListArtistContextPlayCountsForUserParams{
+		ID:      userID,
+		Column2: deviceClass,
+	})
 	if err != nil {
 		return ContextAffinity{}, err
 	}