Gate and meter email group: enterprise license (self-host) + credits (cloud) (#22390)

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.

packages/twenty-server/src/engine/core-modules/billing/app-billing/app-billing.service.ts

@@ -22,6 +22,7 @@ const USAGE_UNIT_BY_OPERATION_TYPE: Record<UsageOperationType, UsageUnit> = {
   [UsageOperationType.CODE_EXECUTION]: UsageUnit.INVOCATION,
   [UsageOperationType.WEB_SEARCH]: UsageUnit.INVOCATION,
   [UsageOperationType.CALL_RECORDING]: UsageUnit.MINUTE,
+  [UsageOperationType.EMAIL_SEND]: UsageUnit.INVOCATION,
 };
 
 // `workspaceId` + `applicationId` come from the application-access token,