Billing - Migrate from Stripe metering (#20298)

**Overall strategy**
**1. Introduce “Billing V2” behind a workspace flag**
Gate the new model with FeatureFlagKey.IS_BILLING_V2_ENABLED so existing
workspaces stay on the old behavior until they’re migrated or explicitly
on V2.

**2. Replace workflow metered SKUs with a resource-credit product**
Conceptually, billable “workflow execution” usage is not the primary
subscription line item anymore. Add a RESOURCE_CREDIT product (and keep
WORKFLOW_NODE_EXECUTION as deprecated for the transition). Usage and
limits are expressed through credit buckets (e.g. price metadata like
credit_amount), so one product can represent pooled credits instead of a
narrow workflow-only meter.

**3. Migrate subscriptions in two layers**
Schema/catalog: persist extra price metadata (instance upgrade) so the
server knows credit amounts and can match Stripe prices to the new
model.
Per workspace: the registered workspace command
upgrade:2-2:migrate-to-billing-v2 finds subscriptions that still have
WORKFLOW_NODE_EXECUTION, swaps those items to the right RESOURCE_CREDIT
prices (using existing Stripe schedule +
BillingSubscriptionUpdateService stack), then treats the workspace as V2
(flag). Workspaces without that legacy item or without a subscription
are skipped.

**4. Unify subscription lifecycle + usage on the server**

**5. Refresh the product surface in Settings**

Test : 

- [x] Subscribe v1 +  Update subscribe + Migrate
- [x]  Subscribe v2 + Update subscribe

packages/twenty-server/src/engine/core-modules/twenty-config/config-variables.ts

@@ -798,6 +798,14 @@ export class ConfigVariables {
   @IsOptional()
   BILLING_USAGE_CAP_CLICKHOUSE_ENABLED = false;
 
+  @ConfigVariablesMetadata({
+    group: ConfigVariablesGroup.BILLING_CONFIG,
+    description: 'Enable billing v2 for new workspaces at checkout',
+    type: ConfigVariableType.BOOLEAN,
+  })
+  @IsOptional()
+  IS_BILLING_V2_ENABLED_FOR_NEW_WORKSPACES = false;
+
   @ConfigVariablesMetadata({
     group: ConfigVariablesGroup.SERVER_CONFIG,
     description: 'Url for the frontend application',