feat(matching): add schedule config for division collision groups

Allow event organizers to configure which divisions run in parallel
(same time slot) for accurate collision detection in the auto-matching
algorithm. Divisions in the same slot will collide with each other.

- Add scheduleConfig JSON field to Event model
- Add PUT /events/:slug/schedule-config API endpoint
- Update matching algorithm to use slot-based collision detection
- Add UI in EventDetailsPage for managing division slots
- Add unit tests for schedule-based collision detection

backend/prisma/schema.prisma

@@ -80,6 +80,7 @@ model Event {
   // Auto-matching configuration
   registrationDeadline DateTime? @map("registration_deadline") // When registration closes
   matchingRunAt        DateTime? @map("matching_run_at")        // When auto-matching was last run
+  scheduleConfig       Json?     @map("schedule_config")        // Division order and collision groups
 
   // Relations
   chatRooms            ChatRoom[]