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>

server/src/app.ts

@@ -11,6 +11,7 @@ import taxesRouter from "./routes/taxes.js";
 import productsRouter from "./routes/products.js";
 import catalogRouter from "./routes/catalog.js";
 import transactionsRouter from "./routes/transactions.js";
+import eventsRouter from "./routes/events.js";
 import { errorHandler } from "./middleware/errorHandler.js";
 import { requestLogger } from "./middleware/requestLogger.js";
 
@@ -40,6 +41,7 @@ export function createApp() {
   app.use("/api/v1/products", productsRouter);
   app.use("/api/v1/catalog", catalogRouter);
   app.use("/api/v1/transactions", transactionsRouter);
+  app.use("/api/v1/events", eventsRouter);
 
   // Serve React admin UI static assets in production
   if (process.env.NODE_ENV === "production") {