feat(android): EventsApi + wire types for /api/events

Retrofit interface + the four request body variants
(play_started / play_ended / play_skipped / play_offline)
matching flutter_client/lib/api/endpoints/events.dart.
play_started's response carries play_event_id (nullable).

Not wired into any caller yet — PlayEventsReporter lands later.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/api/endpoints/EventsApi.kt

@@ -0,0 +1,31 @@
+package com.fabledsword.minstrel.api.endpoints
+
+import com.fabledsword.minstrel.models.wire.PlayEndedRequest
+import com.fabledsword.minstrel.models.wire.PlayOfflineRequest
+import com.fabledsword.minstrel.models.wire.PlaySkippedRequest
+import com.fabledsword.minstrel.models.wire.PlayStartedRequest
+import com.fabledsword.minstrel.models.wire.PlayStartedResponse
+import retrofit2.http.Body
+import retrofit2.http.POST
+
+/**
+ * Retrofit interface for `POST /api/events`. Mirrors the relevant
+ * slice of `flutter_client/lib/api/endpoints/events.dart`. All four
+ * variants share the same URL — the discriminator is in the request
+ * body's `type` field. Server contract is best-effort per spec;
+ * callers (the live path in PlayEventsReporter) swallow errors and
+ * fall back to the offline mutation queue on failure.
+ */
+interface EventsApi {
+    @POST("api/events")
+    suspend fun playStarted(@Body body: PlayStartedRequest): PlayStartedResponse
+
+    @POST("api/events")
+    suspend fun playEnded(@Body body: PlayEndedRequest)
+
+    @POST("api/events")
+    suspend fun playSkipped(@Body body: PlaySkippedRequest)
+
+    @POST("api/events")
+    suspend fun playOffline(@Body body: PlayOfflineRequest)
+}

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

@@ -0,0 +1,51 @@
+package com.fabledsword.minstrel.models.wire
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * Wire shapes for `POST /api/events`. The endpoint multiplexes four
+ * variants on the `type` discriminator field, mirroring
+ * `flutter_client/lib/api/endpoints/events.dart`.
+ *
+ * play_started returns the server-assigned play_event_id (nullable —
+ * server may suppress under certain conditions); the other three
+ * variants return 204-equivalent (no body).
+ */
+
+@Serializable
+data class PlayStartedRequest(
+    val type: String = "play_started",
+    @SerialName("track_id") val trackId: String,
+    @SerialName("client_id") val clientId: String,
+    val source: String? = null,
+)
+
+@Serializable
+data class PlayStartedResponse(
+    @SerialName("play_event_id") val playEventId: String? = null,
+)
+
+@Serializable
+data class PlayEndedRequest(
+    val type: String = "play_ended",
+    @SerialName("play_event_id") val playEventId: String,
+    @SerialName("duration_played_ms") val durationPlayedMs: Long,
+)
+
+@Serializable
+data class PlaySkippedRequest(
+    val type: String = "play_skipped",
+    @SerialName("play_event_id") val playEventId: String,
+    @SerialName("position_ms") val positionMs: Long,
+)
+
+@Serializable
+data class PlayOfflineRequest(
+    val type: String = "play_offline",
+    @SerialName("track_id") val trackId: String,
+    @SerialName("client_id") val clientId: String,
+    val at: String,
+    @SerialName("duration_played_ms") val durationPlayedMs: Long,
+    val source: String? = null,
+)