#50 FTX-50: Policy versioning + snapshot + validation

policies version/status(draft|active|disabled)/key columns; decision stores policy_snapshot JSONB at create (EV-POL-1); apply_action + drainers read snapshot; disabled policy rejects new decisions (EV-POL-2); definition validated per spec 36 schema incl. n_required required when mode=n_of_m (UB-POL-4). Phase 2.