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_device_integration_test.go

@@ -0,0 +1,87 @@
+package recommendation
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+func insertPlayEventDevice(
+	t *testing.T, pool *pgxpool.Pool, userID, trackID pgtype.UUID, startedAt time.Time, device string,
+) {
+	t.Helper()
+	ctx := context.Background()
+	var sessionID pgtype.UUID
+	if err := pool.QueryRow(ctx,
+		`INSERT INTO play_sessions (user_id, started_at, last_event_at, client_id)
+		 VALUES ($1, $2, $2, 'ctx-dev-test') RETURNING id`,
+		userID, startedAt).Scan(&sessionID); err != nil {
+		t.Fatalf("insert play_session: %v", err)
+	}
+	if _, err := pool.Exec(ctx,
+		`INSERT INTO play_events (user_id, track_id, session_id, started_at, was_skipped, device_class)
+		 VALUES ($1, $2, $3, $4, false, $5)`,
+		userID, trackID, sessionID, startedAt, device); err != nil {
+		t.Fatalf("insert play_event: %v", err)
+	}
+}
+
+// TestLoadContextAffinity_DeviceNarrowing seeds one artist played only on
+// mobile and another only on web (all in the current time cell), and verifies
+// the current-device parameter narrows the affinity cell (#1551): on 'mobile'
+// the mobile artist out-scores the web artist and vice-versa, while the
+// device-agnostic (”) pass treats them equally.
+func TestLoadContextAffinity_DeviceNarrowing(t *testing.T) {
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+
+	u := seedUser(t, pool, "ctx-dev")
+	aMobile := seedArtist(t, pool, "MobileArtist", "")
+	aWeb := seedArtist(t, pool, "WebArtist", "")
+	alM := seedAlbumForArtist(t, pool, aMobile.ID, "AlbM")
+	tM := seedTrackOnAlbum(t, pool, alM.ID, aMobile.ID, "TrkM")
+	alW := seedAlbumForArtist(t, pool, aWeb.ID, "AlbW")
+	tW := seedTrackOnAlbum(t, pool, alW.ID, aWeb.ID, "TrkW")
+
+	now := time.Now()
+	for i := 0; i < 5; i++ {
+		insertPlayEventDevice(t, pool, u.ID, tM.ID, now, "mobile")
+		insertPlayEventDevice(t, pool, u.ID, tW.ID, now, "web")
+	}
+
+	mob, err := LoadContextAffinity(ctx, q, u.ID, "mobile")
+	if err != nil {
+		t.Fatalf("mobile: %v", err)
+	}
+	if mob.Affinity(aMobile.ID) <= mob.Affinity(aWeb.ID) {
+		t.Errorf("on mobile, mobile artist (%.3f) should out-score web artist (%.3f)",
+			mob.Affinity(aMobile.ID), mob.Affinity(aWeb.ID))
+	}
+
+	web, err := LoadContextAffinity(ctx, q, u.ID, "web")
+	if err != nil {
+		t.Fatalf("web: %v", err)
+	}
+	if web.Affinity(aWeb.ID) <= web.Affinity(aMobile.ID) {
+		t.Errorf("on web, web artist (%.3f) should out-score mobile artist (%.3f)",
+			web.Affinity(aWeb.ID), web.Affinity(aMobile.ID))
+	}
+
+	// Device-agnostic ('' ): all plays fall in the same time cell, so the two
+	// artists are treated identically — the device dimension is what separates
+	// them above.
+	agn, err := LoadContextAffinity(ctx, q, u.ID, "")
+	if err != nil {
+		t.Fatalf("agnostic: %v", err)
+	}
+	if agn.Affinity(aMobile.ID) != agn.Affinity(aWeb.ID) {
+		t.Errorf("device-agnostic affinities should match, got mobile=%.3f web=%.3f",
+			agn.Affinity(aMobile.ID), agn.Affinity(aWeb.ID))
+	}
+}