Ingest Sessions

Ingest biomech sessions, pitches, and frames.

Idempotency: pitch_uuid is derived server-side from uuid5(NAMESPACE_DNS, "{player_id}:{session_id}:{source_pitch_id}:{pitch_index}"). Callers MUST send the same source_pitch_id + pitch_index for the same physical pitch on every retry; otherwise a retry inserts a new pitch row instead of updating the existing one. pitch_index should be unique within a session.