Milestone M33 - performance improvements and data integrity fixes

Let's go through what we got done in Milestone M33 (Mar 24 - Apr 06).

Enhancements

• Addressed performance and consistency issues reported by the active_record_doctor gem by adding necessary indices, removing unused ones and adding validations at both the database and model levels.

Bug fixes

• Added uniqueness validation to the Vote model to prevent duplicate entries and handle unique constraint violations.