feat: add billing foundation and entitlement enforcement

- add workspace-scoped billing storage, usage tracking, and add-on catalog
- enforce plan entitlements for search and deep research routes
- expand pricing and account UI around billing state, usage, and upgrades

shared/billing/plans.ts

@@ -524,6 +524,18 @@ export function getPublicPricingPlans() {
   return ACTIVE_PLAN_CATALOG.filter((plan) => plan.listingCategory === 'pricing_page_primary');
 }
 
+export function getPublicPricingPlansForInterval(billingInterval: BillingInterval) {
+  return ACTIVE_PLAN_CATALOG.filter((plan) => {
+    if (plan.code === 'enterprise_custom') {
+      return true;
+    }
+
+    return plan.listingCategory === 'pricing_page_hidden'
+      ? plan.billingInterval === billingInterval
+      : plan.listingCategory === 'pricing_page_primary' && plan.billingInterval === billingInterval;
+  });
+}
+
 export function getPlanVariant(tier: PlanTier, billingInterval: BillingInterval) {
   return ACTIVE_PLAN_CATALOG.find((plan) => plan.planFamily === tier && plan.billingInterval === billingInterval) ?? null;
 }