Respect configured lossy conversion bitrate

[TimoStolz]

Problem

When conversion is configured for OPUS with lossy_bitrate = 160, streamrip still passes the codec default bitrate to ffmpeg.

Reproduction:

1. Set conversion to OPUS and lossy_bitrate = 160 in the config.
2. Run rip -vv <url>.
3. The debug output currently reports FFmpeg codec extra argument: -b:a 128k, and the generated ffmpeg command contains -b:a 128k instead of the configured -b:a 160k.

Summary

• pass conversion.lossy_bitrate into lossy converters during track conversion
• make OPUS and AAC generate bitrate ffmpeg args from the configured value
• add conversion tests covering lossy and lossless converter behavior

Tests

• python -m pytest
• python -m black --check streamrip/converter.py streamrip/media/track.py tests/test_converter.py

Note: ruff is not installed in the local environment, so ruff check could not be run here.