fix: transcode non-AAC audio to AAC for MP4 container

[Tingting-Y]

Problem

Generated MP4 files use MP3 audio codec, which is not supported by macOS QuickTime, AVFoundation, and some web browsers. This causes no audio when previewing in web players or QuickTime.

Solution

• Video stream: copy directly, no re-encoding
• AAC audio: copy directly (passthrough)
• MP3/WAV and other non-AAC audio: transcode to AAC 256k

Verification

• 4 unit tests passed
• Python compile check passed
• git diff --check passed
• Real FFmpeg test confirmed:
  • AAC audio is passed through directly
  • MP3/WAV audio is transcoded to AAC 256k
  • Video stream hash is identical across all 3 output types (no re-encoding)