Rename roles, add multi-vendor support, and Events system

Roles: owner→admin, manager→vendor, cashier→user across all routes,
seed, and client UI. Role badge colours updated in UsersPage.

Multi-vendor:
- GET /vendors and GET /users now return all records for admin role;
  vendor/user roles remain scoped to their vendorId
- POST /users: admin can specify vendorId to assign user to any vendor
- vendors/users now include vendor name in responses for admin context

Events (new):
- Prisma schema: Event, EventTax, EventProduct models; Transaction.eventId
- POST/GET/PUT/DELETE /api/v1/events — full CRUD, vendor-scoped
- PUT /events/:id/taxes + DELETE — upsert/remove per-event tax rate overrides
- POST/GET/DELETE /events/:id/products — product allowlist (empty=all)
- GET /events/:id/transactions — paginated list scoped to event
- GET /events/:id/reports/summary — revenue, avg tx, top products for event
- Transactions: eventId accepted in both single POST and batch POST
- Catalog sync: active/upcoming events included in /catalog/sync response

Client:
- Layout nav filtered by role (user role sees Catalog only)
- Dashboard cards filtered by role
- Events page: list, create/edit modal, detail modal with Configuration
  (tax overrides + product allowlist) and Reports tabs

DB: DATABASE_URL updated to file:./prisma/dev.db in .env.example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/App.tsx

@@ -7,6 +7,7 @@ import UsersPage from "./pages/UsersPage";
 import CatalogPage from "./pages/CatalogPage";
 import VendorPage from "./pages/VendorPage";
 import ReportsPage from "./pages/ReportsPage";
+import EventsPage from "./pages/EventsPage";
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const { user, loading } = useAuth();
@@ -43,6 +44,7 @@ export default function App() {
         >
           <Route index element={<DashboardPage />} />
           <Route path="catalog" element={<CatalogPage />} />
+          <Route path="events" element={<EventsPage />} />
           <Route path="users" element={<UsersPage />} />
           <Route path="vendor" element={<VendorPage />} />
           <Route path="reports" element={<ReportsPage />} />