Add database migration tooling (Flyway/Liquibase)

Implement database migration tooling for version-controlled schema changes.

Options:

• Flyway (simpler, SQL-based)
• Liquibase (more features, XML/YAML/SQL)

Tasks:

• Choose migration tool
• Add dependencies
• Configure migration settings
• Create initial migration scripts
• Document migration process
• Add migration validation to CI/CD

Benefits:

• Version-controlled schema changes
• Repeatable deployments
• Easy rollback capability