* Fix Account Holding validation and synchronization
Fixes #1781
- Add comprehensive validations for Account::Holding
- Implement validation to ensure amount matches qty * price
- Update Account::Syncer to include qty and price during synchronization
- Add database constraints for holding attributes and calculations
* Remove database check constraints for Account Holdings
Align with project convention of keeping complex validations in ActiveRecord
- Remove database-level check constraints for quantity, price, and amount
- Maintain database-level null and unique constraints
- Prepare for more flexible validation in the model layer
