List builder webhook destinations

Returns webhook destinations owned by the authenticated API key user. Disabled endpoints are omitted. Subscribable event_types and their payload shapes are described by GET /api/v1/webhooks/events. Four subscribable event types are Pro-only and only deliver to API keys on an active Pro subscription. whale_trades_inserted is one of them, gated by the same SubscriberScope::InsiderOnly mechanism as the other three (each type carries its own LiveEventContract entry; they share the scope value). The other three: wallet_grade_changed (data: wallet, trader_id, old_grade, new_grade, direction (upgrade|downgrade), skill_index, final_score, date) fires on a Pass-2 grade transition; insider_radar_flag_raised (data: trade_id, wallet, trader_id, condition_id, suspicion_score, track, side (yes|no), size, price) fires the first time a trade's suspicion score crosses the radar flag threshold; smart_money_flow_detected (data: condition_id, net_flow_usd, abs_net_flow_usd, dominant_side (yes|no), grade_floor (S|A|B|C|D|F), whale_trade_count, window) fires when a scheduled scanner detects ranked-trader net flow crossing a threshold (up or down) on a market.