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

server/src/account/repository.ts

@@ -1,5 +1,6 @@
 import type { Pool, PoolClient } from 'pg';
 import type { AccountPageData, AccountWorkspace, AppUser, WorkspaceType, WorkspaceRole } from '../../../shared/types.js';
+import { getWorkspaceBillingState } from '../billing/service.js';
 
 type DbClient = Pool | PoolClient;
 
@@ -152,17 +153,13 @@ export async function buildAccountPageData(db: DbClient, user: AppUser): Promise
   }
 
   const summary = await getAccountSummaryForUser(db, user.id);
+  const billing = await getWorkspaceBillingState(db, workspace.id);
 
   return {
     profile: user,
     workspace,
     summary,
-    billing: {
-      status: 'not_configured',
-      planCode: null,
-      billingInterval: null,
-      message: 'Subscription management is being prepared. Plan details, usage tracking, and billing controls will appear here in a future update.',
-    },
+    billing,
     team: {
       canManageMembers: workspace.role === 'owner' || workspace.role === 'admin',
       message: 'Workspace member management is coming soon.',