Design a zero-downtime database migration strategy for a containerized service that cannot tolerate a maintenance window.

Cover: backward-compatible schema migration patterns (additive changes first, deploy code that works with both old and new schema, then remove old schema elements in a later release) rather than a single atomic cutover; using a migration tool integrated into the deployment pipeline (e.g., Flyway/Liquibase for a Spring Boot service) run as a controlled step before the new application version is rolled out; feature-flagging risky migration-dependent behavior so it can be disabled instantly if an issue surfaces; and a clear rollback plan for both the application code and the schema change indepen