fix(converter): disable copy_art for Vorbis and OPUS to prevent FFmpeg muxer error

[berettavexee]

Problem

Converting to OGG (Vorbis) or OPUS fails with a cryptic error when the source file contains a cover art video stream:

ERROR Error downloading track: FFmpeg output: (None, b'')

The root cause: when a FLAC or MP3 source has embedded artwork, FFmpeg includes a video stream. The Converter base class then adds -c:v copy to the FFmpeg command — but the OGG and Opus muxers do not support video streams, so FFmpeg exits with an error.

A secondary bug compounded this: Vorbis and OPUS defined copy_art = False as class attributes, but Converter.__init__ always overwrote them with self.copy_art = copy_art (default True), so the class attribute was silently ignored.

Fix

Two changes:

1. Vorbis and OPUS: add copy_art = False class attribute to opt out of artwork copying.
2. Converter.__init__: change copy_art parameter default from True to Optional[bool] = None, and use the class attribute as the fallback when no value is passed explicitly:

   self.copy_art = getattr(type(self), "copy_art", True) if copy_art is None else copy_art

   This lets subclasses opt out via a class attribute without being overridden by the constructor default.

Test plan

• rip -c ogg url <url> — no FFmpeg error, .ogg files produced
• rip -c opus url <url> — no FFmpeg error, .opus files produced
• rip -c mp3 url <url> — unchanged behaviour (artwork still embedded)
• rip -c aac url <url> — unchanged behaviour (artwork still embedded)

🤖 Generated with Claude Code

[berettavexee]

Superseded by #992, which replaces the copy_art = False workaround with a proper fix: -vn to discard video in the FFmpeg command, and mutagen post-conversion embedding via METADATA_BLOCK_PICTURE.