Introduce member.subscription_updated webhook

Problem Statement

Today, subscription lifecycle events (create, update, cancel, expire) do not consistently trigger a clear, subscription-specific member update webhook, which results in stale or inaccurate member data, incorrect active subscription states, and missed or misfired marketing automations.

User Story

As a subscription marketing team,

I want Pelcro to emit a dedicated member update webhook whenever subscription lifecycle events occur,

So that downstream systems like Sailthru always receive accurate, subscription-driven member updates and can reliably trigger renewals, expirations, and subscription marketing workflows.

Definition of Done (DoD)

1. A new webhook event (subscription.member_updated) is introduced to represent member updates triggered by subscription lifecycle changes.

2. The webhook is triggered when a subscription is:

3. renewed or updated (including phase additions or plan changes)

4. canceled (immediate or at period end)

5. expired