Create billing customer at signup so onboarding rewards are credited (#22633)

## Problem

Onboarding credit rewards (install apps, import contacts, invite team)
were silently dropped. They credit the workspace balance via
`billingCustomer.increment(...)`, but no `billingCustomer` row exists
until the plan step (it's created lazily when the first subscription is
set up, which is after those steps). So the increment affected 0 rows
and the credit was lost. A user installing 3 apps saw only the trial
grant, not the expected +1.5 credits.

## Fix

Create the Stripe customer + `billingCustomer` row eagerly at signup via
a new `BillingCreditService.ensureBillingCustomer`, called from
`signUpOnNewWorkspace` after the workspace transaction commits. It is
idempotent, guarded by `IS_BILLING_ENABLED`, and non-blocking (failures
are logged, not thrown). The later subscription flow reuses this
customer (no duplicate Stripe customer), and trial eligibility is
unchanged since the customer has no subscriptions yet.

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packages/twenty-server/src/engine/core-modules/auth/services/sign-in-up.service.spec.ts

@@ -108,6 +108,9 @@ const createSignInUpServiceForTests = () => {
     {
       creditWorkspaceBalance: jest.fn(),
     } as any,
+    {
+      isBillingEnabled: jest.fn(),
+    } as any,
     {
       createQueryRunner: jest.fn(() => queryRunnerMock),
     } as any,

packages/twenty-server/src/engine/core-modules/auth/services/sign-in-up.service.ts

@@ -25,6 +25,7 @@ import {
 } from 'src/engine/core-modules/app-token/app-token.entity';
 import { ApplicationService } from 'src/engine/core-modules/application/application.service';
 import { BillingCreditService } from 'src/engine/core-modules/billing/services/billing-credit.service';
+import { BillingService } from 'src/engine/core-modules/billing/services/billing.service';
 import {
   AuthException,
   AuthExceptionCode,
@@ -93,6 +94,7 @@ export class SignInUpService {
     private readonly enterprisePlanService: EnterprisePlanService,
     private readonly eventLogEmitterService: EventLogEmitterService,
     private readonly billingCreditService: BillingCreditService,
+    private readonly billingService: BillingService,
     @InjectDataSource()
     private readonly dataSource: DataSource,
   ) {}
@@ -708,6 +710,14 @@ export class SignInUpService {
         .createContext({ workspaceId })
         .insertWorkspaceEvent(WORKSPACE_CREATED_EVENT, {});
 
+      if (this.billingService.isBillingEnabled()) {
+        await this.billingService.ensureBillingCustomer({
+          userEmail: email,
+          workspaceId: workspace.id,
+          workspaceDisplayName: workspace.displayName,
+        });
+      }
+
       return { user, workspace };
     } catch (error) {
       const isSubdomainConflict =