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/usage/enums/usage-operation-type.enum.ts

@@ -9,6 +9,7 @@ export enum UsageOperationType {
   CODE_EXECUTION = 'CODE_EXECUTION',
   WEB_SEARCH = 'WEB_SEARCH',
   CALL_RECORDING = 'CALL_RECORDING',
+  EMAIL_SEND = 'EMAIL_SEND',
 }
 
 registerEnumType(UsageOperationType, {

packages/twenty-server/src/engine/core-modules/usage/enums/usage-resource-type.enum.ts

@@ -7,4 +7,5 @@ export enum UsageResourceType {
   STORAGE = 'STORAGE',
   API = 'API',
   LOGIC_FUNCTION = 'LOGIC_FUNCTION',
+  EMAIL = 'EMAIL',
 }