Fix Drift indexes and preparation ordering

[jjoonleo]

Summary

• Add Drift indexes for schedule/preparation lookup fields and a schema 4 upgrade path that creates them for existing schema 3 databases.
• Reuse the domain preparation ordering logic in schedule/user DAOs for O(n) reconstruction with broken-link and cycle safety.
• Add Preparation/Schedule glossary terms and tests for DAO ordering, corrupted links, cycles, and index migration expectations.

Closes #541
Related: #392

Tests

• dart run build_runner build --delete-conflicting-outputs
• flutter analyze
• flutter test test/core/database/database_index_test.dart test/data/daos/preparation_schedule_dao_test.dart test/data/daos/preparation_user_dao_test.dart test/data/daos/schedule_dao_test.dart
• flutter test (704 tests passed)

Notes

• Drift generated files were regenerated locally but remain ignored by the repository .gitignore.