MED-876 AC2 / IV3 — replace the In Use prompt selections doc

Replaces this coach's coach_session_prep.ai_prompt_selections JSONB document atomically. The document is keyed by agenda segment block_key; each segment carries independent boolean toggles for the three primary prompt slots (purpose, speaking, stall_recovery) and an optional speaking_override set when the coach picked one of the two ALTERNATIVE PROMPT rows via "Use This". The AC1 banner count and the AC4 PDF inclusion both derive from this document. Rejected (409 AI_FEATURES_DISABLED) when the cohort's modality has ai_features_enabled=false.