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

frontend/src/services/api.js

@@ -292,7 +292,8 @@ export const heatsAPI = {
 
   async getMyHeats(slug) {
     const data = await fetchAPI(`/events/${slug}/heats/me`);
-    return data.data;
+    // Returns { data: heats[], competitorNumber: number|null }
+    return data;
   },
 
   async getAllHeats(slug) {
@@ -306,6 +307,14 @@ export const heatsAPI = {
     });
     return data;
   },
+
+  async setCompetitorNumber(slug, competitorNumber) {
+    const data = await fetchAPI(`/events/${slug}/competitor-number`, {
+      method: 'PUT',
+      body: JSON.stringify({ competitorNumber }),
+    });
+    return data;
+  },
 };
 
 // Matches API (Phase 2)