feat(events): add competitor number (bib) support

Allow participants to set their bib/competitor number per event.
Display as badge next to username in participant lists.

- Add competitorNumber field to EventParticipant model
- Add PUT /events/:slug/competitor-number endpoint
- Include competitorNumber in heats/me and heats/all responses
- Add input field in HeatsBanner component
- Display badge in UserListItem component
- Add unit tests for competitor number feature

backend/prisma/schema.prisma

@@ -182,10 +182,11 @@ model Rating {
 
 // Event participants (tracks which users joined which events)
 model EventParticipant {
-  id        Int      @id @default(autoincrement())
-  userId    Int      @map("user_id")
-  eventId   Int      @map("event_id")
-  joinedAt  DateTime @default(now()) @map("joined_at")
+  id               Int      @id @default(autoincrement())
+  userId           Int      @map("user_id")
+  eventId          Int      @map("event_id")
+  competitorNumber Int?     @map("competitor_number") // Bib number - one per user per event
+  joinedAt         DateTime @default(now()) @map("joined_at")
 
   // Relations
   user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)