fix(teacher): register migration, containment period queries, concurrency guards

- Register AppDatabase.MIGRATION_3_4 so v3 databases upgrade without data loss
- Query teacher schedule periods by containment (weekStart<=x AND weekEnd>=x)
  instead of exact weekStart, matching 2-week catalog windows
- Clip buildTeacherSchedule days to the requested week range
- Add weeksFor() to enumerate weeks of a catalog window for the UI selector
- Serialize syncPeriod body with a Mutex to protect the unique index
- Rethrow CancellationException before generic catch; version loadRevision
  checks so stale observers/loads are cancelled
- Sync job tracking in PickerViewModel; keep loading true while empty
- Drop dead selectTeacher helper

app/src/main/java/agzam/vyatsu/ui/picker/PickerViewModel.kt

@@ -13,6 +13,7 @@ import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import java.time.DayOfWeek
 import java.time.temporal.TemporalAdjusters
+import kotlin.coroutines.cancellation.CancellationException
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
@@ -45,6 +46,7 @@ class PickerViewModel(
     val uiState: StateFlow<PickerUiState> = _uiState.asStateFlow()
 
     private var teachersJob: kotlinx.coroutines.Job? = null
+    private var syncJob: kotlinx.coroutines.Job? = null
 
     init {
         groupsLoad()
@@ -139,11 +141,20 @@ class PickerViewModel(
 
         _uiState.value = _uiState.value.copy(syncProgress = null, teachersLoading = true)
 
-        viewModelScope.launch {
-            runCatching {
+        syncJob?.cancel()
+        syncJob = viewModelScope.launch {
+            try {
                 teacherScheduleRepository.syncPeriod(today).collect { progress ->
                     _uiState.value = _uiState.value.copy(syncProgress = progress)
                 }
+            } catch (e: CancellationException) {
+                throw e
+            } catch (e: Throwable) {
+                _uiState.value = _uiState.value.copy(
+                    error = e.message ?: "Не удалось загрузить расписание преподавателей"
+                )
+            } finally {
+                _uiState.value = _uiState.value.copy(teachersLoading = false)
             }
         }
 
@@ -151,10 +162,11 @@ class PickerViewModel(
         teachersJob = viewModelScope.launch {
             teacherScheduleRepository.observeTeachers(period).collect { teachers ->
                 val query = _uiState.value.teacherQuery
+                val isLoading = _uiState.value.teachersLoading
                 _uiState.value = _uiState.value.copy(
-                    teachersLoading = false,
                     teachers = teachers,
                     filteredTeachers = filterTeachers(teachers, query),
+                    teachersLoading = isLoading && teachers.isEmpty(),
                     error = null
                 )
             }
@@ -178,11 +190,4 @@ class PickerViewModel(
     fun retryTeachers() {
         syncAndObserveTeachers()
     }
-
-    fun selectTeacher(teacher: Teacher) {
-        viewModelScope.launch {
-            preferencesRepository.clearSelectedGroup()
-            preferencesRepository.setSelectedTeacherId(teacher.name)
-        }
-    }
 }