feat(android): MinstrelPlayerService — Media3 MediaSessionService (M8 phase 6.2)

The actual replacement for everything audio_service plugin wrapped.
Media3 owns the foreground-service lifecycle, MediaSession token,
notification card, lock-screen surface, Bluetooth/AVRCP routing,
Pixel Watch tile, and Android Auto adapter natively — no plugin
layer between us and the platform.

Service shape (~25 LOC):
  - @AndroidEntryPoint MediaSessionService
  - @Inject PlayerFactory builds ExoPlayer in onCreate
  - onGetSession returns the live MediaSession to any binding
    controller (system UI, Wear OS companion, MediaController3 clients)
  - onTaskRemoved keeps playing while audio is active (standard
    media-app behavior); otherwise stopSelf so notification clears
  - onDestroy releases session + player

Compare with flutter_client/lib/player/audio_handler.dart's 1000+ LOC
across MinstrelAudioHandler + the soft-teardown / stall-watchdog /
recovery machinery. Media3 owns most of that natively; we'll get to
the small portions we still need (queue management, position
reporting facade) in 6.3.

Manifest registration: foregroundServiceType="mediaPlayback" +
MediaSessionService intent-filter. MediaButtonReceiver is registered
by the Media3 library; no manual receiver class needed (Flutter's
manifest had to declare audio_service's receiver explicitly).

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

android/app/src/main/AndroidManifest.xml

@@ -31,6 +31,13 @@
             </intent-filter>
         </activity>
 
-        <!-- Media3 MediaSessionService is registered in Phase 6 (Task 6.2). -->
+        <service
+            android:name=".player.MinstrelPlayerService"
+            android:exported="true"
+            android:foregroundServiceType="mediaPlayback">
+            <intent-filter>
+                <action android:name="androidx.media3.session.MediaSessionService" />
+            </intent-filter>
+        </service>
     </application>
 </manifest>