feat: add QR code event check-in system

Backend:
- Add event_checkin_tokens table with unique tokens per event
- Implement GET /api/events/:slug/details endpoint (on-demand token generation)
- Implement POST /api/events/checkin/:token endpoint (date validation only in production)
- Implement DELETE /api/events/:slug/leave endpoint
- Add comprehensive test suite for check-in endpoints

Frontend:
- Add EventDetailsPage with QR code display, participant list, and stats
- Add EventCheckinPage with success/error screens
- Add "Leave Event" button with confirmation modal to EventChatPage
- Install qrcode.react library for QR code generation
- Update routing and API client with new endpoints

Features:
- QR codes valid from (startDate-1d) to (endDate+1d)
- Development mode bypasses date validation for testing
- Automatic participant count tracking
- Duplicate check-in prevention
- Token reuse for same event (generated once, cached)

backend/prisma/schema.prisma

@@ -75,10 +75,24 @@ model Event {
   chatRooms         ChatRoom[]
   matches           Match[]
   participants      EventParticipant[]
+  checkinToken      EventCheckinToken?
 
   @@map("events")
 }
 
+// Event check-in tokens (QR code tokens for event access)
+model EventCheckinToken {
+  id        Int      @id @default(autoincrement())
+  eventId   Int      @unique @map("event_id")
+  token     String   @unique @default(cuid()) @db.VarChar(50)
+  createdAt DateTime @default(now()) @map("created_at")
+
+  // Relations
+  event     Event    @relation(fields: [eventId], references: [id], onDelete: Cascade)
+
+  @@map("event_checkin_tokens")
+}
+
 // Chat rooms (event chat and private 1:1 chat)
 model ChatRoom {
   id        Int       @id @default(autoincrement())