Handle SubscriptionChange events with consent-aware suppression transitions

Problem/Motivation

SubscriptionChange is currently stored without the fields needed to interpret it, and the suppression helper does not evaluate it. Unsubscribe/manual suppression and reactivation therefore have no explicit state model.

Evidence and scope

Reviewed 1.0.0-alpha1, source commit 02fd9d36af5237e712cecb7155d79725f7824880. Location: src/Controller/PostmarkWebhookController.php:89.

Provider documentation: https://postmarkapp.com/developer/webhooks/subscription-change-webhook . It describes stream-scoped changes and SuppressSending, including events without MessageID. Current code ignores those state fields.

Proposed resolution

Parse a minimal set of SubscriptionChange fields and apply explicit, ordered transitions. Distinguish hard-bounce recovery from complaint or recipient opt-out; never treat any reactivation as blanket permission to erase consent restrictions. Build on source scope and durable state.

Acceptance criteria

Fixtures cover suppress/reactivate, missing MessageID, out-of-order events and each supported reason. Specify precedence and retain minimal audit evidence without storing raw payloads.