Expose payment_method_status, institutional_member & seat_status in webhook payloads

Problem Statement

As a Pelcro integration developer, I experience three required fields being unavailable from the Pelcro webhook payloads when building ESP and CRM syncs, which results in either dropping the field from the integration or making an extra authenticated API call per event — added latency, rate-limit pressure, and a per-integration workaround instead of one platform fix.

User Story

As a Pelcro integration developer, I want payment_method_status, institutional_member and seat_status to arrive on the existing customer and subscription webhook payloads, so that any downstream integration can map them directly with no supplementary API calls.

Definition of Done

A feature is done when:

• Given a subscription with a payment method, when a subscription.* webhook fires, then the payload's payment method / source object includes status (chargeable / pending / failed / ...) matching that payment method's actual state in Pelcro.

• Given a customer holding an institutional or membership seat, when a customer.* or subscription.* webhook fires, then the payload includes institutional_member and seat_status, resolvable without a supplementary API call. [TO BE DETERMINED] — exact semantics pending client clarification, see Open Questions.

• This change will impact the API (webhook payload serialization in pelcro-core — app/WebHooks/Resources/, including SourceResource). No SDK, UI, or Integration changes in this task.

• This solution will include the following limitations: additive fields only, no existing field renamed, removed or retyped; no backfill or replay of historical webhook events; seat_status semantics for individual (non-membership) subscriptions remain undefined until the client confirms.