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-client-sdk/src/metadata/generated/schema.graphql

@@ -1484,6 +1484,7 @@ enum BillingUsageType {
 """The different billing products available"""
 enum BillingProductKey {
   BASE_PRODUCT
+  RESOURCE_CREDIT
   WORKFLOW_NODE_EXECUTION
 }
 
@@ -1492,6 +1493,7 @@ type BillingPriceLicensed {
   unitAmount: Float!
   stripePriceId: String!
   priceUsageType: BillingUsageType!
+  creditAmount: Float
 }
 
 enum SubscriptionInterval {
@@ -1590,7 +1592,8 @@ type BillingMeteredProductUsage {
 
 type BillingPlan {
   planKey: BillingPlanKey!
-  licensedProducts: [BillingLicensedProduct!]!
+  baseProducts: [BillingLicensedProduct!]!
+  resourceCreditProducts: [BillingLicensedProduct!]!
   meteredProducts: [BillingMeteredProduct!]!
 }
 
@@ -1754,6 +1757,7 @@ enum FeatureFlagKey {
   IS_RICH_TEXT_V1_MIGRATED
   IS_RECORD_PAGE_LAYOUT_GLOBAL_EDITION_ENABLED
   IS_DATASOURCE_MIGRATED
+  IS_BILLING_V2_ENABLED
 }
 
 type WorkspaceUrls {