Add module management system for enabling/disabling features

Stores can enable/disable feature modules from Settings. When disabled,
nav links are hidden and API routes return 403. Designed as the
foundation for future license-based gating (licensed + enabled flags).

Core modules (Accounts, Members, Users, Roles, Settings) are always on.

- module_config table with slug, name, description, licensed, enabled
- In-memory cache for fast per-request module checks
- requireModule middleware wraps route groups in main.ts
- Settings page Modules card with toggle switches
- Sidebar hides nav links for disabled modules
- Default modules seeded: inventory, pos, repairs, rentals, lessons,
  files, vault, email, reports

packages/admin/src/api/modules.ts

@@ -0,0 +1,28 @@
+import { queryOptions } from '@tanstack/react-query'
+import { api } from '@/lib/api-client'
+
+export interface ModuleConfig {
+  id: string
+  slug: string
+  name: string
+  description: string | null
+  licensed: boolean
+  enabled: boolean
+  createdAt: string
+  updatedAt: string
+}
+
+export const moduleKeys = {
+  list: ['modules'] as const,
+}
+
+export function moduleListOptions() {
+  return queryOptions({
+    queryKey: moduleKeys.list,
+    queryFn: () => api.get<{ data: ModuleConfig[] }>('/v1/modules'),
+  })
+}
+
+export const moduleMutations = {
+  toggle: (slug: string, enabled: boolean) => api.patch<ModuleConfig>(`/v1/modules/${slug}`, { enabled }),
+}