bind the download engine by its interface in Koin

`single { SystemDownloadEngine(...) }` registers the concrete class, but
DownloadRepository asks for the DownloadEngine interface, so resolving
PlaybackController threw NoDefinitionFoundException and the app died on launch
with a Koin InstanceCreationException. It is now `single<DownloadEngine>`, the
way the other interface singletons in the module are declared.

Nothing in the suite ever started Koin — the repository tests build the
repository directly with a fake — which is why a launch-crashing binding got
through. AppModuleTest now resolves the playback graph from the real module, and
it failed with the same NoDefinitionFoundException before this fix.

app/src/main/kotlin/com/subtracks/di/AppModule.kt

@@ -6,6 +6,7 @@ import android.os.Handler
 import android.os.Looper
 import android.widget.Toast
 import com.subtracks.data.db.createAndroidDatabase
+import com.subtracks.data.download.DownloadEngine
 import com.subtracks.data.download.SystemDownloadEngine
 import com.subtracks.data.net.networkMode
 import com.subtracks.data.prefs.createUserPreferences
@@ -48,7 +49,7 @@ fun appModule(
     single<ArtworkSeedStore> { ArtworkSeedRepository(get()) }
     single { SyncManager(get(), get(), get()) }
     single { QueueRepository(get()) }
-    single { SystemDownloadEngine(context.applicationContext) }
+    single<DownloadEngine> { SystemDownloadEngine(context.applicationContext) }
     single {
         DownloadRepository(
             db = get(),