refactor(backend): drop TranscodeRunner pass-through, call MediaTranscoder directly

backend/src/main/kotlin/dev/twango/jetplay/rpc/MediaAccessorImpl.kt

@@ -10,8 +10,8 @@ import dev.twango.jetplay.media.MediaInfo
 import dev.twango.jetplay.media.Spectrogram
 import dev.twango.jetplay.transcode.FfmpegAvailability
 import dev.twango.jetplay.transcode.MediaInfoExtractor
+import dev.twango.jetplay.transcode.MediaTranscoder
 import dev.twango.jetplay.transcode.SpectrogramExtractor
-import dev.twango.jetplay.transcode.TranscodeRunner
 import dev.twango.jetplay.transcode.WaveformExtractor
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.Flow
@@ -84,7 +84,7 @@ class MediaAccessorImpl : MediaAccessor {
             val output = try {
                 withContext(Dispatchers.IO) {
                     // onProgress fires synchronously inside ffmpeg, so trySend (non-suspending) bridges it.
-                    TranscodeRunner.transcode(input) { pct -> trySend(TranscodeEvent.Progress(pct)) }
+                    MediaTranscoder.transcode(input) { pct -> trySend(TranscodeEvent.Progress(pct)) }
                 }
             } catch (e: Exception) {
                 send(TranscodeEvent.Failed(e.message ?: "unknown"))

backend/src/main/kotlin/dev/twango/jetplay/transcode/MediaTranscoder.kt

@@ -39,6 +39,7 @@ object MediaTranscoder {
     /** Must stay in sync with the shared classifier. */
     internal val rawAudioExtensions: Set<String> get() = RAW_AUDIO_HINTS.keys
 
+    /** Runs ffmpeg, invoking onProgress(percent). Returns the transcoded File; throws on failure. */
     fun transcode(inputFile: File, onProgress: (Double) -> Unit = {}): File {
         val outputFile = Files.createTempFile("jetplay-", ".webm").toFile().apply { deleteOnExit() }