Fix Account Holding validation and synchronization (#1818)

* 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

.cursor/rules/project-conventions.mdc

@@ -76,3 +76,8 @@ Due to the open-source nature of this project, we have chosen Minitest + Fixture
 - Keep fixtures to a minimum.  Most models should have 2-3 fixtures maximum that represent the "base cases" for that model.  "Edge cases" should be created on the fly, within the context of the test which it is needed.
 - For tests that require a large number of fixture records to be created, use Rails helpers such as [entries_test_helper.rb](mdc:test/support/account/entries_test_helper.rb) to act as a "factory" for creating these.  For a great example of this, check out [balance_calculator_test.rb](mdc:test/models/account/balance_calculator_test.rb)
 
+### Convention 7: Use ActiveRecord for complex validations, DB for simple ones, keep business logic out of DB
+
+- Enforce `null` checks, unique indexes, and other simple validations in the DB
+- ActiveRecord validations _may_ mirror the DB level ones, but not 100% necessary.  These are for convenience when error handling in forms.  Always prefer client-side form validation when possible.
+- Complex validations and business logic should remain in ActiveRecord