fix(android): always serialize event type so plays aren't dropped

The native client's /api/events requests omitted the `type`
discriminator entirely. The app's Json is configured with
encodeDefaults=false (AppModule), so a `type` left at its data-class
default ("play_started" etc.) is never written to the wire. The server
multiplexes on `type` and returns 400 "unknown event type" for an
empty one, which PlayEventsReporter's catch swallows — and the
play_started path has no offline fallback, so the play is lost with no
trace.

Net effect: EVERY native Android play event (started/ended/skipped/
offline) has 400'd since this code was written. Listening History only
ever populated from the Flutter/web clients; as usage moved to the
native app, History went sparse. Confirmed live in the server access
log: POST /api/events -> 400 on every play, while reads 200.

Force the discriminator onto the wire with
@EncodeDefault(Mode.ALWAYS) on each request type's `type` field.
Surgical (vs flipping encodeDefaults globally), and idiomatic for a
constant-valued discriminator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -1,5 +1,9 @@
+@file:OptIn(ExperimentalSerializationApi::class)
+
 package com.fabledsword.minstrel.models.wire
 
+import kotlinx.serialization.EncodeDefault
+import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
@@ -15,7 +19,11 @@ import kotlinx.serialization.Serializable
 
 @Serializable
 data class PlayStartedRequest(
-    val type: String = "play_started",
+    // Discriminator MUST stay on the wire. The app's Json has
+    // encodeDefaults=false, so without this a `type` left at its
+    // default is omitted entirely and the server 400s the event as
+    // "unknown event type" — silently dropping every native play.
+    @EncodeDefault(EncodeDefault.Mode.ALWAYS) val type: String = "play_started",
     @SerialName("track_id") val trackId: String,
     @SerialName("client_id") val clientId: String,
     val source: String? = null,
@@ -28,21 +36,21 @@ data class PlayStartedResponse(
 
 @Serializable
 data class PlayEndedRequest(
-    val type: String = "play_ended",
+    @EncodeDefault(EncodeDefault.Mode.ALWAYS) 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",
+    @EncodeDefault(EncodeDefault.Mode.ALWAYS) 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",
+    @EncodeDefault(EncodeDefault.Mode.ALWAYS) val type: String = "play_offline",
     @SerialName("track_id") val trackId: String,
     @SerialName("client_id") val clientId: String,
     val at: String,