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/types.ts

@@ -1,4 +1,5 @@
-import type { BillingInterval, PlanCode } from './billing/plans.js';
+import type { AddonCode, BillingInterval, PlanCode } from './billing/plans.js';
+import type { UsageAllowanceAvailability, UsageResource } from './billing/entitlements.js';
 
 export type JobStatus = 'pending' | 'running' | 'completed' | 'failed' | 'stopped';
 
@@ -34,10 +35,30 @@ export interface AccountSummary {
 
 export type AccountBillingStatus = 'not_configured' | 'inactive' | 'active' | 'past_due' | 'canceled';
 
+export interface BillingUsageResourceSummary {
+  resource: UsageResource;
+  availability: UsageAllowanceAvailability;
+  included: number | null;
+  consumed: number;
+  remaining: number | null;
+}
+
+export interface BillingAddonBalanceSummary {
+  addonCode: AddonCode;
+  resource: UsageResource;
+  remainingQuantity: number;
+  expiresAt: string | null;
+}
+
 export interface AccountBillingState {
   status: AccountBillingStatus;
   planCode: PlanCode | null;
   billingInterval: BillingInterval | null;
+  currentPeriodStartsAt: string | null;
+  currentPeriodEndsAt: string | null;
+  cancelAtPeriodEnd: boolean;
+  usage: BillingUsageResourceSummary[];
+  addonBalances: BillingAddonBalanceSummary[];
   message: string;
 }