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>

backend/app/main.py

@@ -32,6 +32,7 @@ async def lifespan(app: FastAPI):
         await _add_column_if_missing(conn, "schedule_templates", "day_start_time", "TIME NULL")
         await _add_column_if_missing(conn, "schedule_templates", "day_end_time", "TIME NULL")
         await _add_column_if_missing(conn, "schedule_blocks", "duration_minutes", "INT NULL")
+        await _add_column_if_missing(conn, "children", "strikes", "INT NOT NULL DEFAULT 0")
     yield