feat: add audio engine — Opus encoder, play queue with 4 modes, FFmpeg player

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/audio/encoder.ts

@@ -0,0 +1,25 @@
+import { OpusEncoder } from "@discordjs/opus";
+
+const SAMPLE_RATE = 48000;
+const CHANNELS = 2;
+const FRAME_DURATION_MS = 20;
+export const FRAME_SIZE = (SAMPLE_RATE * FRAME_DURATION_MS) / 1000; // 960 samples
+export const PCM_FRAME_BYTES = FRAME_SIZE * CHANNELS * 2; // 3840 bytes (16-bit stereo)
+
+export interface Encoder {
+  encode(pcm: Buffer): Buffer;
+  decode(opus: Buffer): Buffer;
+}
+
+export function createOpusEncoder(): Encoder {
+  const opus = new OpusEncoder(SAMPLE_RATE, CHANNELS);
+
+  return {
+    encode(pcm: Buffer): Buffer {
+      return opus.encode(pcm);
+    },
+    decode(opusData: Buffer): Buffer {
+      return opus.decode(opusData);
+    },
+  };
+}