refactor: 연관관계 수정(#12)

app.js

config/sequelize.js

controllers/scheduleController.js

dtos/FriendListDTO.js

dtos/FriendResponseDTO.js

dtos/MeetingDetailResponseDTO.js

dtos/MeetingResponseDTO.js

dtos/ScheduleRequestDTO.js

dtos/ScheduleResponseDTO.js

jest.setup.js

models/Friend.js

models/meeting.js

models/meetingParticipant.js

models/user.js

package-lock.json

package.json

services/friendService.js

test/friendService.test.js

test/schedule.test.js