[Production readiness] Complete and test Drift schema migrations

[jjoonleo]

Summary

The Drift database schema is version 3, but the migration strategy only handles upgrades from version 1. There is no explicit path for users upgrading from version 2 to version 3.

Evidence

• File: lib/core/database/database.dart
• schemaVersion => 3.
• onUpgrade checks only if (from == 1) and creates preparationSchedules and preparationUsers.
• No migration branch handles from == 2 or validates intermediate migrations.

Why this is not production ready

• Existing users can hit failed or incomplete local database migrations after app update.
• Local schedule/preparation data may become unavailable or corrupted.
• Migration bugs often appear only after release on real user devices.

Scope

Create complete, tested Drift migrations for all supported schema upgrade paths.

Suggested implementation

• Determine what changed between schema versions 1, 2, and 3.
• Implement stepwise migrations for 1 -> 2, 2 -> 3, and 1 -> 3 as appropriate.
• Add migration tests with old schema fixtures or Migrator verification.
• Confirm web Drift storage behavior if web persistence is supported.

Acceptance criteria

• Upgrades from every supported previous schema version to current schema succeed.
• Migration tests cover all expected upgrade paths.
• Foreign keys remain enabled after migration.
• Existing local data is preserved or intentionally transformed with documentation.

Project priority

P0 - User data safety blocker.