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/events.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/jackc/pgx/v5"
@@ -24,6 +25,33 @@ type eventRequest struct {
 	// / "discover" feed the system-playlist rotation dedup (#415);
 	// absent / "" for library, user-playlist, radio, Subsonic.
 	Source *string `json:"source"`
+	// DeviceClass is the client's self-reported device class (#1551),
+	// normalized + stored per play to condition radio on the current device.
+	DeviceClass *string `json:"device_class"`
+}
+
+// knownDeviceClasses whitelists the device classes the context-affinity facet
+// buckets by (#1551). A client sends one; anything unrecognized normalizes to
+// "other", and empty/absent to "" (stored NULL → excluded from the device
+// dimension). Kept permissive (no DB CHECK) so a new client class is one
+// whitelist entry, not a migration.
+var knownDeviceClasses = map[string]bool{
+	"mobile": true, "tablet": true, "desktop": true,
+	"web": true, "tv": true, "watch": true,
+}
+
+func normalizeDeviceClass(dc *string) string {
+	if dc == nil {
+		return ""
+	}
+	s := strings.ToLower(strings.TrimSpace(*dc))
+	if s == "" {
+		return ""
+	}
+	if knownDeviceClasses[s] {
+		return s
+	}
+	return "other"
 }
 
 type playStartedResponse struct {
@@ -114,7 +142,9 @@ func (h *handlers) handleEventPlayStarted(
 	if req.Source != nil {
 		source = *req.Source
 	}
-	res, err := h.events.RecordPlayStartedWithSource(r.Context(), user.ID, trackID, clientID, source, at)
+	deviceClass := normalizeDeviceClass(req.DeviceClass)
+	res, err := h.events.RecordPlayStartedWithSource(
+		r.Context(), user.ID, trackID, clientID, source, deviceClass, at)
 	if err != nil {
 		h.logger.Error("api: events: play_started", "err", err)
 		writeErr(w, apierror.InternalMsg("record failed", err))
@@ -159,7 +189,8 @@ func (h *handlers) handleEventPlayOffline(
 		source = *req.Source
 	}
 	if err := h.events.RecordOfflinePlay(
-		r.Context(), user.ID, trackID, clientID, source, at, *req.DurationPlayedMs,
+		r.Context(), user.ID, trackID, clientID, source,
+		normalizeDeviceClass(req.DeviceClass), at, *req.DurationPlayedMs,
 	); err != nil {
 		h.logger.Error("api: events: play_offline", "err", err)
 		writeErr(w, apierror.InternalMsg("record failed", err))