- Implement chatFileService(db) with create, getById, listByConversation, listByMessage, createReference, listReferences, attachToMessage
- Export deriveCategory() helper mapping MIME types to image/code/document/other
- Add unit tests verifying service methods and category derivation with mocked DB
- Add ChatFile, ChatFileReference, ChatFileUploadResponse, ChatFileListResponse interfaces to types/chat.ts
- Add optional files?: ChatFile[] field to ChatMessage interface
- Add uploadChatFileSchema and createFileReferenceSchema Zod validators to validators/chat.ts
- Re-export all new types and validators from shared/src/index.ts
- Create test stubs for chat-file-service.test.ts and chat-file-routes.test.ts with it.todo() entries
