[nooby6]

Summary

• Add explicit, date/time availability windows per employee with authenticated create/list endpoints.
• Require new and updated scheduled shifts to fit wholly inside one configured availability window; employees with no configured windows retain existing behaviour during rollout, and existing shifts are grandfathered when availability is first configured.
• Enforce shift-window checks in PostgreSQL as well as the service layer, using a per-employee transaction advisory lock to serialize availability/shift writes and prevent check-then-write races.
• Map availability violations to a stable EMPLOYEE_AVAILABILITY_CONFLICT 409 response and add service regression tests for create/update violations.

Validation

• GitHub Actions passed on the initial implementation. A follow-up migration refinement preserves existing schedules when availability is configured; CI will rerun for the updated commit.