Issue #25: Create user item relation

src/db/relations/user-item.relation.ts

src/db/repo/items.repo.ts

src/db/repo/user-item.repo.ts

src/db/schema/index.ts

src/index.ts

src/routes/items.route.ts

src/routes/user-item.route.ts

src/services/items.service.ts

src/services/user-item.relation.service.ts