Adds mailcheep integration to synchronize contacts with mailcheep's email
marketing platform. Contacts are synced on user creation and during stripe
subscription lifecycle events (paid, churned).
- Add MailcheepService for mailcheep API interactions
- Add SyncMailcheepContactJob for async contact synchronization
- Add sync:mailcheep console command for backfilling contacts
- Integrate contact syncing into user creation workflow
- Integrate contact syncing into stripe subscription event handling
- Add mailcheep configuration (API key and list IDs)
- Add comprehensive feature tests for mailcheep integration