[zfleeman]

Fixes #127 and #128. Both bugs were in the same few lines that read the input's audio bitrate.

• MKV input crashes with KeyError: 'bit_rate' #127: MKV files (OBS's default format) don't report a bitrate for each stream, so ff4d crashed with KeyError: 'bit_rate'. When the bitrate is missing, ff4d now uses 96 kbps, the same as the -a default (FALLBACK_AUDIO_BR).
• Default audio bitrate comes from the last audio track instead of the first #128: _parse_streams() remembered the last audio track's index, so a clip with game audio and a mic track took its default bitrate from the mic. ff4d now reads it from self.audio_streams[0], so _parse_streams() no longer needs to return anything.

The default bitrate only applies when no audio bitrate is set, for example a blank Audio Bitrate field in the web UI.

Testing

• Added a test for each bug. Both fail without the fix.
• uv run pytest --cov: 292 passed, 99.39% coverage. ruff format and ruff check are clean.
• Real encode of a 2-second MKV with two audio tracks: it used to crash on startup. It now encodes with -b:a 96000.