Email group (marketing email) was gated only by the
`IS_EMAIL_GROUP_ENABLED` feature flag with no server-side enforcement.
This adds real gating, split by deployment:
- **Self-hosted** (`IS_BILLING_ENABLED=false`): requires a valid
Enterprise plan.
- **Cloud** (billing enabled): metered by credits, mirroring the
existing AI credit system. Priced on AWS SES cost ($0.10/1,000 outbound)
× 3 margin = $0.30/1,000 (300 micro-credits/email). Pre-flight blocks
sends when out of credits; each email is charged after SES accepts it,
in the async send job — matching how AWS bills us (no refund on bounce).
Enforcement is applied at every email group resolver, and denials
surface as proper client errors through a dedicated GraphQL exception
filter.
neo773
6f64be5751