[Feat] 컬렉션 기본 CRUD API 구현

[kangcheolung]

📌 Description

권한 블록 - collections / collection_documents 기본 CRUD API 구현 (F-PERM-01, 02)

✅ To-do

• CollectionRepository, CollectionDocumentRepository
• CollectionConverter
• CollectionCommandService, CollectionQueryService
• CollectionController
• DTO: CreateCollectionRequest, CollectionResponse, AddDocumentRequest, CollectionDocumentResponse
• POST /collections
• GET /collections/{id}
• POST /collections/{collectionId}/documents
• ErrorCode: COLLECTION_NOT_FOUND, DOCUMENT_NOT_FOUND

📒 기타

• 엔티티(DocumentCollection, CollectionDocument)와 Flyway 마이그레이션(V7, V8)은 이미 완료됨
• documents 테이블은 A 담당자 영역, FK 읽기 전용 참조

[kangcheolung]

Branch feature/16 created!