feat: Implement database migration system

[xDefcon]

Changes Made

• Added database migration system Implemented a structured migration system to handle database schema evolution systematically.
• Initial Schema (_createDB) is now handled as a migration
• Added docs to the database migration system

How It Works

The migration system automatically handles schema changes through versioned migration classes:

1. Each migration has a version number and description
2. The MigrationManager tracks database versions and applies migrations in sequence
3. New migrations are registered in migration_registry.dart
4. When upgrading the database, only migrations with newer version numbers are applied in the specified order

This approach provides a reliable way to evolve the database schema while maintaining backward compatibility with existing data.

Additional Considerations

• Migration Rollback: Rollback functionality is not implemented by purpose (database versions can only go up). To modify a previous migration, create a new one with the corrective changes.
• Logging: Currently using print() statements wrapped with kDebugMode checks for development visibility. A future enhancement should implement a proper logging system (using the logger package or a custom logging service which is not the purpose of this PR)
• Migration Testing: Each migration should be tested independently to ensure it works correctly both on fresh installs and when upgrading from previous versions. Maybe add some tests related to this?

[xDefcon]

Related to issue #261

@mikev-cw Could you review this? I'd also appreciate your feedback on the additional considerations mentioned above. Thanks!

This PR requires testing, possibly including automated tests.

[mikev-cw]

Super, of course I will. I think this could be a valuable addition and i want to go super deep this contribution though! :D Give me some time. @xDefcon are you on our Discord, in case i need some clarifications?

[xDefcon]

@mikev-cw Yes I am, with the same username.

[mikev-cw]

@xDefcon I'm merging this, also without tests. We need to do a release next days and we want to have this feature ready for that. Please, if you can, create an issue for tests. Thank you so much!