v1.0.9

v1.0.9 — remix single-stem-solo

Soloing one stem in /v1/audio/remix (stem_mix={"vocals":1.0,
"drums":0.0,"bass":0.0,"other":0.0}) used to fail with
"mix_audio requires at least 2 inputs" — the post-separation mix
step couldn't handle a single surviving stem.

Fix: new audio.apply_gain_db() helper (single-stream ffmpeg
volume= pass); remix handler short-circuits to it when exactly
one stem survives gain filtering.

483/483 CUDA integration tests passing.