perf(teacher): lower sync memory footprint

- Reduce sync parallelism 5 -> 2 to cap peak heap while bulk-fetching
  all departments for a period
- Replace per-write rows (insert schedule/days/lessons separately) with a
  single @Transaction replaceSchedule() so Room invalidates observers once
  per department instead of ~4x each, cutting main-thread recompositions
  and intermediate allocations during the 38-department sync

app/src/main/java/agzam/vyatsu/data/local/TeacherScheduleDao.kt

@@ -4,6 +4,7 @@ import androidx.room.Dao
 import androidx.room.Insert
 import androidx.room.OnConflictStrategy
 import androidx.room.Query
+import androidx.room.Transaction
 import kotlinx.coroutines.flow.Flow
 
 @Dao
@@ -38,6 +39,16 @@ interface TeacherScheduleDao {
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun insertLessons(lessons: List<TeacherLessonEntity>)
 
+    @Transaction
+    suspend fun replaceSchedule(schedule: TeacherScheduleEntity, days: List<TeacherDayEntity>, lessons: List<TeacherLessonEntity>) {
+        val scheduleId = insertSchedule(schedule)
+        val daysWithScheduleId = days.map { it.copy(scheduleId = scheduleId) }
+        deleteDaysForSchedule(scheduleId)
+        val dayIds = insertDays(daysWithScheduleId)
+        val dayIdByDate = daysWithScheduleId.zip(dayIds).associate { (day, id) -> day.date to id }
+        insertLessons(lessons.map { lesson -> lesson.copy(dayId = dayIdByDate.getValue(lesson.date)) })
+    }
+
     @Query(
         "SELECT teacher_days.* FROM teacher_days INNER JOIN teacher_schedules ON teacher_schedules.id = teacher_days.scheduleId " +
             "WHERE teacher_schedules.season = :season AND teacher_schedules.weekStart <= :weekStart " +