[nooby6]

Summary

• Reject shift updates that overlap approved employee leave, not just other scheduled shifts.
• Return a stable 409 EMPLOYEE_INACTIVE response instead of allowing inactive-employee scheduling to surface as a server error.
• Expand service tests for shift conflicts, approved leave conflicts, inactive employees, invalid windows, adjacent windows, and successful scheduling.

Validation

• Added targeted unit coverage; GitHub Actions CI will run the full Gradle test suite, including the PostgreSQL concurrency integration test.

Closes #3