Repaint: local audio region regeneration

[thedavidweng]

Problem

No way to regenerate a portion of an existing audio file. Users must regenerate from scratch to iterate on a section.

Scope

• Waveform drag-select to define a region for regeneration
• ACE-Step /edit endpoint integration for local audio region replacement
• UI for repaint parameters (region, new prompt, strength)

Acceptance criteria

• User can select a region on the waveform
• Selected region can be regenerated with a new prompt via /edit
• Repainted audio replaces or appends to the original file
• CLI supports repaint via openloop run --repaint <file> --start <s> --end <s>

[thedavidweng]

Backend feasibility confirmed: ACE-Step 1.5 (commit d5d958e) supports repainting via the same /release_task endpoint. The GenerateMusicRequest model includes repainting_start, repainting_end, src_audio_path, repaint_mode, repaint_strength, cover_noise_strength, repaint_latent_crossfade_frames, and repaint_wav_crossfade_sec.\n\nCurrent OpenLoop code hardcodes task_type="text2music" and leaves src_audio_path as None, so repaint is not reachable. This issue should focus on exposing the existing backend repaint capability: select an audio region, set task_type appropriately, and submit src_audio_path + region params.