Ingest Baselines

Ingest per-player biomech baselines and baseline frames.

New baselines are inserted normally. Replacing an existing baseline requires replace_existing=true; only then does the endpoint DELETE all existing biomech_baseline_frame rows for that player_id and bulk-insert the new ones. skip_existing=true leaves existing baselines unchanged. Each baseline is committed independently.

Player resolution: same caveat as /historical-stats — if player.alias triggers bootstrap, the new player_info row will have team_id=NULL. Establish player_info via POST /biomech/sessions or roster creation first to avoid teamless pitcher records.