[Feature] Mailchimp ↔ Stripe integration + consent capture

The CRM plumbing every lifecycle email depends on. Currently the email issues exist with no foundation to build on. Foundation dependency for the 📣 Affiliate & Marketing category.

### Scope
- [ ] Connect Stripe events (purchase, subscription start/lapse, abandoned checkout) → Mailchimp audiences/tags.
- [ ] Wire mailing-list signup (site + in-game sign-up flow) into Mailchimp.
- [ ] Consent capture and storage (lawful basis, unsubscribe, preference centre) — ties to #4029.
- [ ] Audience segmentation: subscribers, lapsed, purchasers, never-purchased.
- [ ] Define the event → audience → trigger map used by downstream flows.

### Dependencies
- Blocks: #4029 (consent on sign-up), #4046 (abandoned cart), #4047 (retention sequences).
- Related: #4048 (NPS — survey results into Mailchimp segments).

Parent: #4027