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/api/radio.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"log/slog"
@@ -81,6 +82,9 @@ func (h *handlers) handleRadio(w http.ResponseWriter, r *http.Request) {
 	}
 
 	currentVec := loadCurrentSessionVector(r, q, user.ID, h.logger)
+	// Condition on the current device (#1551): the latest play's device is a
+	// cheap, request-free proxy for what the user is on right now.
+	currentVec.DeviceClass = latestDeviceClass(r.Context(), q, user.ID, h.logger)
 
 	exclude := parseExcludeParam(r.URL.Query().Get("exclude"))
 	limits := recommendation.DefaultCandidateSourceLimits()
@@ -150,6 +154,24 @@ func loadCurrentSessionVector(r *http.Request, q *dbq.Queries, userID pgtype.UUI
 	return v
 }
 
+// latestDeviceClass returns the device_class of the user's most recent play as
+// the "current device" for context conditioning (#1551), or "" when unknown
+// (no plays yet, or the latest play predates device capture). Best-effort: a
+// lookup failure yields a device-agnostic ("") affinity cell.
+func latestDeviceClass(ctx context.Context, q *dbq.Queries, userID pgtype.UUID, logger *slog.Logger) string {
+	dc, err := q.GetLatestPlayDeviceClassForUser(ctx, userID)
+	if err != nil {
+		if !errors.Is(err, pgx.ErrNoRows) {
+			logger.Warn("api: radio: latest device class", "err", err)
+		}
+		return ""
+	}
+	if dc == nil {
+		return ""
+	}
+	return *dc
+}
+
 // parseExcludeParam parses a comma-separated list of UUIDs from the
 // `exclude` query string, silently dropping malformed entries. Returns
 // nil for empty or all-malformed input.