feat: reschedule reservation

[heitorrsdev]

Description

Implements the "Reschedule Reservation" feature end-to-end, from domain logic to HTTP endpoint, including documentation and tests. The feature allows a client or barber to update the start/end times of an existing active reservation via PATCH /reservations/:id.

What was done

• Added two new domain errors: CannotRescheduleCancelledReservationError and CannotReschedulePastReservationError
• Added reschedule() method to the Reservation entity, enforcing all business invariants (cancelled status, authorization, past reservation, 1-hour client notice, invalid time range)
• Made startTime and endTime mutable via private backing fields with public getters
• Created RescheduleReservationCommand and RescheduleReservationUseCase
• Registered the use case in ApplicationModule
• Added RescheduleReservationDto for request validation
• Exposed PATCH /reservations/:id in ReservationController
• Mapped new errors in DomainErrorHttpMapper
• Added 7 unit tests for reservation.reschedule() covering all invariants
• Added 5 E2E tests for the PATCH endpoint
• Added UC-09 to docs/use-cases.md

Why

Completing the core reservation lifecycle (create → cancel → reschedule) is a critical domain feature. The implementation follows the established layered architecture: domain invariants are validated in the entity, orchestration happens in the use case, and conflict detection is delegated to the PostgreSQL exclusion constraint (same proven pattern as CreateReservationUseCase).