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/prisma/schema.prisma

@@ -20,11 +20,12 @@ model Vendor {
   products     Product[]
   taxes        Tax[]
   transactions Transaction[]
+  events       Event[]
 }
 
 model Role {
   id    String @id @default(cuid())
-  name  String @unique // cashier | manager | owner
+  name  String @unique // admin | vendor | user
 
   users User[]
 }
@@ -74,8 +75,9 @@ model Tax {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
-  vendor   Vendor    @relation(fields: [vendorId], references: [id])
-  products Product[]
+  vendor         Vendor     @relation(fields: [vendorId], references: [id])
+  products       Product[]
+  eventOverrides EventTax[]
 }
 
 model Product {
@@ -96,6 +98,7 @@ model Product {
   category         Category?         @relation(fields: [categoryId], references: [id])
   tax              Tax?              @relation(fields: [taxId], references: [id])
   transactionItems TransactionItem[]
+  eventProducts    EventProduct[]
 }
 
 model Transaction {
@@ -103,6 +106,7 @@ model Transaction {
   idempotencyKey String   @unique
   vendorId       String
   userId         String
+  eventId        String?
   status         String   // pending | completed | failed | refunded
   paymentMethod  String   // cash | card
   subtotal       Float
@@ -115,6 +119,7 @@ model Transaction {
 
   vendor Vendor            @relation(fields: [vendorId], references: [id])
   user   User              @relation(fields: [userId], references: [id])
+  event  Event?            @relation(fields: [eventId], references: [id])
   items  TransactionItem[]
 }
 
@@ -132,3 +137,48 @@ model TransactionItem {
   transaction Transaction @relation(fields: [transactionId], references: [id])
   product     Product     @relation(fields: [productId], references: [id])
 }
+
+// ─── Events ───────────────────────────────────────────────────────────────────
+
+model Event {
+  id          String   @id @default(cuid())
+  vendorId    String
+  name        String
+  description String?
+  startsAt    DateTime
+  endsAt      DateTime
+  isActive    Boolean  @default(true)
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  vendor       Vendor         @relation(fields: [vendorId], references: [id])
+  taxOverrides EventTax[]
+  products     EventProduct[]
+  transactions Transaction[]
+}
+
+// Tax rate overrides for a specific event. Shadows the vendor-level Tax for the
+// event duration. Empty = use vendor defaults.
+model EventTax {
+  id      String @id @default(cuid())
+  eventId String
+  taxId   String
+  rate    Float  // override rate in percent
+
+  event Event @relation(fields: [eventId], references: [id], onDelete: Cascade)
+  tax   Tax   @relation(fields: [taxId], references: [id])
+
+  @@unique([eventId, taxId])
+}
+
+// Allowlist of products available at an event. Empty = all vendor products available.
+model EventProduct {
+  id        String @id @default(cuid())
+  eventId   String
+  productId String
+
+  event   Event   @relation(fields: [eventId], references: [id], onDelete: Cascade)
+  product Product @relation(fields: [productId], references: [id])
+
+  @@unique([eventId, productId])
+}