Move 3 Strikes from Admin to Dashboard, add strikes feature

- Adds strikes (0-3) to Child model with migration
- New PATCH /api/children/{id}/strikes endpoint with WebSocket broadcast
- TV dashboard shows red ✕ marks next to child name when strikes > 0
- 3 Strikes card on Dashboard page (removed from Admin)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/schedule.js

@@ -55,6 +55,10 @@ export const useScheduleStore = defineStore('schedule', () => {
       applySnapshot(event)
       return
     }
+    if (event.event === 'strikes_update') {
+      if (child.value) child.value = { ...child.value, strikes: event.strikes }
+      return
+    }
     // Session ended
     if (event.is_active === false) {
       session.value = null