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/di/AppContainer.kt

@@ -23,6 +23,7 @@ class AppContainer(context: Context) {
         AppDatabase::class.java,
         "vyatsu_schedule.db"
     )
+        .addMigrations(AppDatabase.MIGRATION_3_4)
         .fallbackToDestructiveMigration(dropAllTables = true)
         .build()