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
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