feat(story-1.3): add user registration API with Argon2 password hashing

internal/domain/user.go

@@ -0,0 +1,26 @@
+// Package domain contains business logic entities and interfaces.
+package domain
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// User represents a registered user in the system.
+type User struct {
+	ID           uuid.UUID `json:"id"`
+	Email        string    `json:"email"`
+	PasswordHash string    `json:"-"` // Never expose in JSON
+	IsAdmin      bool      `json:"is_admin"`
+	CreatedAt    time.Time `json:"created_at"`
+	UpdatedAt    time.Time `json:"updated_at"`
+}
+
+// UserRepository defines the interface for user data access.
+type UserRepository interface {
+	Create(user *User) error
+	GetByEmail(email string) (*User, error)
+	GetByID(id uuid.UUID) (*User, error)
+	Exists(email string) (bool, error)
+}