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>

android/app/src/main/java/com/fabledsword/minstrel/cache/mutations/MutationQueue.kt

@@ -239,6 +239,9 @@ data class PlayOfflinePayload(
     val atIso: String,
     val durationPlayedMs: Long,
     val source: String? = null,
+    // #1551: device class for context conditioning; null on payloads queued
+    // before this field existed (decodes to null → server stores NULL).
+    val deviceClass: String? = null,
 )
 
 /**

android/app/src/main/java/com/fabledsword/minstrel/cache/mutations/MutationReplayer.kt

@@ -254,6 +254,7 @@ class MutationReplayer @Inject constructor(
                 at = decoded.atIso,
                 durationPlayedMs = decoded.durationPlayedMs,
                 source = decoded.source,
+                deviceClass = decoded.deviceClass,
             ),
         )
         return Outcome.SENT

android/app/src/main/java/com/fabledsword/minstrel/models/wire/EventsWire.kt

@@ -27,6 +27,8 @@ data class PlayStartedRequest(
     @SerialName("track_id") val trackId: String,
     @SerialName("client_id") val clientId: String,
     val source: String? = null,
+    // #1551: device class for context conditioning (server normalizes).
+    @SerialName("device_class") val deviceClass: String? = null,
 )
 
 @Serializable
@@ -56,4 +58,6 @@ data class PlayOfflineRequest(
     val at: String,
     @SerialName("duration_played_ms") val durationPlayedMs: Long,
     val source: String? = null,
+    // #1551: device class for context conditioning (server normalizes).
+    @SerialName("device_class") val deviceClass: String? = null,
 )

android/app/src/main/java/com/fabledsword/minstrel/player/PlayEventsReporter.kt

@@ -155,6 +155,7 @@ class PlayEventsReporter @Inject constructor(
                         trackId = trackId,
                         clientId = cid,
                         source = source.takeIf { !it.isNullOrEmpty() },
+                        deviceClass = DEVICE_CLASS,
                     ),
                 )
                 if (curTrackId == trackId) {
@@ -248,6 +249,7 @@ class PlayEventsReporter @Inject constructor(
                     atIso = startedAt.toString(),
                     durationPlayedMs = durationPlayedMs,
                     source = source.takeIf { !it.isNullOrEmpty() },
+                    deviceClass = DEVICE_CLASS,
                 ),
             )
         }
@@ -268,6 +270,12 @@ class PlayEventsReporter @Inject constructor(
 
     // ── Lifecycle: durable-close on app background / detach ────────
 
+    companion object {
+        // #1551: Android is a mobile client. Refine to tablet/tv via device
+        // configuration later if the metrics trend view shows it matters.
+        private const val DEVICE_CLASS = "mobile"
+    }
+
     override fun onStop(owner: LifecycleOwner) {
         // App backgrounded. If the tracked play is still PLAYING, leave it
         // alone: the foreground media service keeps the process — and this