Stories Domain: Entity + DTO + Mapper

[ArchILLtect]

Summary

Add StoryEntity, StoryDto, CreateStoryRequest, and StoryMapper.

Motivation / Problem

Establish domain and API models for the Stories MVP with strict separation of entity vs DTO.

Acceptance Criteria

• StoryEntity with fields: id, authorId, content, createdAt, updatedAt; @Table(name="stories"); timestamps via @PrePersist/@PreUpdate.
• DTOs: StoryDto { id, authorId, content, createdAt, updatedAt }, CreateStoryRequest { content } with Bean Validation @NotBlank @Size(min=1, max=1000).
• StoryMapper entity ↔ dto helpers.
• No controller returns JPA entities directly (DTO only).

Target Release

v0.1.2

Notes / Links

docs/weekly-plan/week-5/week-5-plan-backend.md
docs/weekly-plan/week-5/week-5-plan-issues.md