You have inherited a 10-year-old monolithic application written in Java with no unit tests and high coupling. The business requirements are changing rapidly. Outline a step-by-step refactoring strategy to introduce modularity and testability without halting feature development. Discuss the 'Strangler Fig' pattern, the importance of the 'Seam' concept, and how to establish a characterization test suite. Prioritize the technical debt items you would address first and justify your choices.