5. One-to-one, one-to-many, many-to-many

// One-to-many: Customer 1 — * Order
b.Entity<Order>().HasOne(o => o.Customer).WithMany(c => c.Orders)
 .HasForeignKey(o => o.CustomerId).OnDelete(DeleteBehavior.Restrict);

// One-to-one: User 1 — 1 Profile (dependent has the FK)
b.Entity<User>().HasOne(u => u.Profile).WithOne(p => p.User).HasForeignKey<Profile>(p => p.UserId);

// Many-to-many (EF Core 5+): skip navigations, join table created automatically
public class Student { public int Id { get; set; } public List<Course> Courses { get; set; } = []; }
public class Course  { public int Id { get; set; } public List<Student> Students { get; set; } = []; }

// Many-to-many with payload → explicit join entity
public class Enrollment { public int StudentId { get; set; } public int CourseId { get; set; } public DateTime EnrolledOn { get; set; } }