Bound separation input WAV duration to the probed length

The unbounded apad filter made ffmpeg pad the 3-second test clip
endlessly (a 22.9GB WAV) until the disk filled, which was the real
cause of the voice separation failures. Trim to the probed duration
and drop apad.

app/api/canvas/audio-separate/route.ts

@@ -58,14 +58,15 @@ export async function POST(request: Request) {
     const info = await probeMedia(input, 600);
     if (!info.hasAudio) throw new Error("当前视频没有音轨");
     const inputAudio = path.join(dir, "separation-input.wav");
+    const durationSeconds = Math.round(info.duration / 1e6);
     await ffmpeg(
       [
         ...mediaSafety,
         "-i",
         input,
         "-vn",
-        "-af",
-        "apad",
+        "-t",
+        String(durationSeconds),
         "-ac",
         "2",
         "-ar",