Add challenges background preference support: Extend site preferences and related components to include challengesBackground, update API and UI to handle new background image settings for challenges.

prisma/schema.prisma

@@ -99,6 +99,7 @@ model SitePreferences {
   homeBackground        String?
   eventsBackground      String?
   leaderboardBackground String?
+  challengesBackground  String?
   createdAt             DateTime @default(now())
   updatedAt             DateTime @updatedAt
 }