feat(billing): add formula A/B admin UI and subscription renew status

Expose dual metering formula editing with batch user assignment, show
auto-renew and pending-recharge states in subscriptions, and document
billing acceptance checks for production rollout.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/api/client.ts

@@ -887,6 +887,7 @@ export async function updateAdminUser(
     balanceCents: number;
     spaceQuotaBytes: number;
     role: 'user' | 'admin';
+    billingFormula: 'A' | 'B';
   }>,
 ): Promise<PortalUser> {
   const result = await portalFetch<{ user: PortalUser }>(`/admin-api/users/${userId}`, {
@@ -1409,16 +1410,28 @@ export async function syncSubscriptionPlansToProduction(): Promise<PlanSyncResul
   return result.sync;
 }
 
-export async function getBillingConfig(): Promise<BillingAdminConfigResponse> {
-  return portalFetch('/admin-api/billing/config');
+export async function getBillingConfig(formula: 'A' | 'B' = 'A'): Promise<BillingAdminConfigResponse> {
+  const q = formula === 'B' ? '?formula=B' : '';
+  return portalFetch(`/admin-api/billing/config${q}`);
 }
 
 export async function updateBillingConfig(
   config: BillingAdminConfig,
+  formula: 'A' | 'B' = 'A',
 ): Promise<BillingAdminConfigResponse> {
   return portalFetch('/admin-api/billing/config', {
     method: 'PUT',
-    body: JSON.stringify({ config }),
+    body: JSON.stringify({ config, formula }),
+  });
+}
+
+export async function assignBillingFormula(
+  userIds: string[],
+  formula: 'A' | 'B',
+): Promise<{ ok: boolean; updated: number; formula: 'A' | 'B'; message?: string }> {
+  return portalFetch('/admin-api/billing/formula-assignments', {
+    method: 'POST',
+    body: JSON.stringify({ userIds, formula }),
   });
 }