Implement "Register Guest" (UC-020) in a Hexagonal Multi-Module Backend

Problem Description

A hotel booking system's backend is a Maven reactor at backend/ with five modules — hotel-domain, hotel-business, hotel-postgres, hotel-api, hotel-app — following a hexagonal (ports and adapters) architecture. Only one feature exists so far, RoomType, fully wired across every module:

• hotel-domain: com.example.hotel.domain.roomtype.RoomType — a pure record, zero framework imports.
• hotel-business: com.example.hotel.business.roomtype.RoomTypeService (concrete @Service), RoomTypeRepository (the one real port — a plain interface sitting next to the service, not in a port subpackage), dto.RoomTypeDTO (a record with a static fromBusiness(RoomType) factory method).
• hotel-postgres: postgres.roomtype.model.RoomTypeEntity (separate JPA @Entity), postgres.roomtype.converter.{RoomTypeConverter,RoomTypeEntityConverter} (hand-written static converters), postgres.roomtype.query.RoomTypeJpaRepository (Spring Data JpaRepository), postgres.roomtype.RoomTypeRepositoryImpl (implements the business module's port), and the Flyway migration V1__create_room_type.sql.
• hotel-api: com.example.hotel.api.roomtype.RoomTypeController — a @RestController that calls the concrete RoomTypeService directly. There is no inbound port/use-case interface anywhere in this reactor — the controller depends on the service class, not an interface.

The entity model is at docs/entity_model.md (includes both RoomType, already implemented, and Guest, not yet implemented). The use case to implement is specified at docs/use-cases/UC-020-register-guest.md.

Output Specification

Implement UC-020 "Register Guest" by following the exact same pattern as the existing RoomType feature, across the same five modules:

1. hotel-domain: a pure Guest record (no framework imports).
2. hotel-business: a concrete GuestService, a GuestRepository port interface (plain sibling file, matching RoomTypeRepository's placement — do not put it in a port subpackage), and a GuestDTO record with a static fromBusiness(Guest) factory method.
3. hotel-postgres: a GuestEntity JPA entity, hand-written static converters (matching the RoomTypeConverter/RoomTypeEntityConverter naming and shape — do not use MapStruct or any other mapping library), a GuestJpaRepository Spring Data interface, a GuestRepositoryImpl implementing the business module's port, and a new Flyway migration (V2__create_guest.sql) in this module's own src/main/resources/db/migration — enforce the email uniqueness from BR-020 at the database level too.
4. hotel-api: a GuestController that calls the concrete GuestService directly — do not create an inbound port/use-case interface for this feature; none exists anywhere in the reactor today, so match that.

Do not create any test files or test classes — that is out of scope for this task.