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/repository/TeacherScheduleRepositoryImpl.kt

@@ -156,33 +156,15 @@ class TeacherScheduleRepositoryImpl(
         val html = VyatsuHttpClient.getBytes(url).toString(Charsets.UTF_8)
         val parsed = TeacherScheduleParser.parse(html)
 
-        val scheduleId = scheduleDao.insertSchedule(
-            TeacherScheduleEntity(
-                departmentId = target.departmentId,
-                season = season,
-                weekStart = target.weekStart,
-                weekEnd = target.weekEnd,
-                lastSyncedAt = now()
-            )
-        )
-        replaceDays(scheduleId, parsed.days)
-    }
-
-    private suspend fun replaceDays(scheduleId: Long, days: List<Day>) {
-        scheduleDao.deleteDaysForSchedule(scheduleId)
-        val dayEntities = days.map {
-            TeacherDayEntity(scheduleId = scheduleId, date = it.date.toEpochDay())
+        val dayEntities = parsed.days.map {
+            TeacherDayEntity(scheduleId = 0, date = it.date.toEpochDay())
         }
-        val dayIds = scheduleDao.insertDays(dayEntities)
-        val dayIdByDate = dayEntities.zip(dayIds).associate { (day, id) ->
-            LocalDate.ofEpochDay(day.date) to id
-        }
-        val lessons = days.flatMap { day ->
+        val lessonEntities = parsed.days.flatMap { day ->
             day.intervals.flatMapIndexed { slotIndex, slot ->
                 slot.lessons.mapIndexedNotNull { lessonIndex, lesson ->
                     val teacher = lesson.teacher ?: return@mapIndexedNotNull null
                     TeacherLessonEntity(
-                        dayId = dayIdByDate.getValue(day.date),
+                        dayId = 0,
                         date = day.date.toEpochDay(),
                         slotIndex = slotIndex,
                         start = slot.start.toMinutesOfDay(),
@@ -199,7 +181,17 @@ class TeacherScheduleRepositoryImpl(
                 }
             }
         }
-        scheduleDao.insertLessons(lessons)
+        scheduleDao.replaceSchedule(
+            TeacherScheduleEntity(
+                departmentId = target.departmentId,
+                season = season,
+                weekStart = target.weekStart,
+                weekEnd = target.weekEnd,
+                lastSyncedAt = now()
+            ),
+            dayEntities,
+            lessonEntities
+        )
     }
 
     private suspend fun trimToRecent(keepDistinctPeriods: Int) {
@@ -262,7 +254,7 @@ class TeacherScheduleRepositoryImpl(
 
     companion object {
         private const val STALE_WINDOW_MS = 12 * 60 * 60 * 1000L
-        private const val PARALLELISM = 5
+        private const val PARALLELISM = 2
         private const val TAG = "TeacherScheduleRepo"
         private val RANGE_FMT = java.time.format.DateTimeFormatter.ofPattern("ddMMyyyy")
     }