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/migrations/20251121210620_add_competitor_number/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "event_participants" ADD COLUMN     "competitor_number" INTEGER;