Create a builder webhook destination

Creates a pending HTTPS webhook destination. The response includes one-time signing_secret and verification.token values. Deliveries are not sent until the endpoint is verified. The subscribable event_types and their data payload shapes are described by GET /api/v1/webhooks/events; the per-endpoint delivery log is GET /api/v1/webhooks/{id}/deliveries. 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. Delivery signing: each delivery request carries an HMAC-SHA256 signature in the x-0xinsider-signature header formatted as v1=<hex>, where <hex> is HMAC-SHA256(signing_secret, "<timestamp>.<raw_request_body>"). The signed <timestamp> is sent separately as x-0xinsider-timestamp (unix seconds). To verify a delivery: read x-0xinsider-timestamp, reject it if it differs from the current time by more than 300 seconds, recompute v1=<hex> over "<timestamp>.<raw_body>" with your signing_secret, and compare against x-0xinsider-signature using a constant-time comparison. Each delivery also carries x-0xinsider-event-id, x-0xinsider-event-type, x-0xinsider-delivery-id, and x-0xinsider-delivery-attempt headers.