Implement Hexagonal Architecture Pattern

### Description

**Priority:** Medium
**Type:** Refactoring
**Dependencies:** Upgrade Java and Spring Boot Versions

**Description:**
The current architecture has tight coupling between components, making testing difficult and limiting flexibility. Implementing the Hexagonal Architecture (Ports and Adapters) pattern will improve separation of concerns and testability.

**Acceptance Criteria:**
- [ ] Refactor core domain logic to be independent of infrastructure concerns
- [ ] Define port interfaces for all external dependencies (database, SMS service, etc.)
- [ ] Implement adapters for each port that connect to actual infrastructure
- [ ] Update dependency injection to wire ports to their adapters
- [ ] Ensure all existing functionality is preserved
- [ ] Verify improved testability by writing tests that use mock adapters

**Technical Notes:**
Start with one bounded context (e.g., OTP management) and gradually refactor others. This architectural change will make the system more maintainable and testable. Consider using package structure to enforce the hexagonal architecture, with separate packages for domain, ports, and adapters.