server: implement wav/mp3 audio download

If set, will save audio transcription to disk.
MP3 conversion is on-request, but cached to disk as well only if it is successfull.

Closes #148

server/reflector/processors/audio_file_writer.py

@@ -0,0 +1,35 @@
+from reflector.processors.base import Processor
+import av
+import wave
+from pathlib import Path
+
+
+class AudioFileWriterProcessor(Processor):
+    """
+    Write audio frames to a file.
+    """
+
+    INPUT_TYPE = av.AudioFrame
+    OUTPUT_TYPE = av.AudioFrame
+
+    def __init__(self, path: Path | str):
+        super().__init__()
+        if isinstance(path, str):
+            path = Path(path)
+        self.path = path
+        self.fd = None
+
+    async def _push(self, data: av.AudioFrame):
+        if not self.fd:
+            self.path.parent.mkdir(parents=True, exist_ok=True)
+            self.fd = wave.open(self.path.as_posix(), "wb")
+            self.fd.setnchannels(len(data.layout.channels))
+            self.fd.setsampwidth(data.format.bytes)
+            self.fd.setframerate(data.sample_rate)
+        self.fd.writeframes(data.to_ndarray().tobytes())
+        await self.emit(data)
+
+    async def _flush(self):
+        if self.fd:
+            self.fd.close()
+            self.fd = None