Add JWT authentication

- Create register and login use cases
- Implement JWT service for token generation and validation
- Add authentication middleware to protect habit endpoints
- Create auth HTTP handlers (register, login)
- Update habit handlers to extract userID from JWT token
- Register/login endpoints: POST /auth/register, POST /auth/login
- Habit endpoints now require Bearer token in Authorization header
- Tested: register -> create habit -> list habits works correctly

internal/application/commands/register_user.go

@@ -0,0 +1,58 @@
+package commands
+
+import (
+	"context"
+
+	"habit-tracker-api/internal/domain/entities"
+	"habit-tracker-api/internal/domain/repositories"
+	"habit-tracker-api/internal/shared/errors"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+type RegisterUserCommand struct {
+	Email    string
+	Password string
+	Timezone string
+}
+
+type RegisterUserHandler struct {
+	userRepo repositories.UserRepository
+}
+
+func NewRegisterUserHandler(userRepo repositories.UserRepository) *RegisterUserHandler {
+	return &RegisterUserHandler{userRepo: userRepo}
+}
+
+func (h *RegisterUserHandler) Handle(ctx context.Context, cmd RegisterUserCommand) (string, error) {
+	if cmd.Email == "" || cmd.Password == "" {
+		return "", errors.ErrInvalidInput
+	}
+
+	if len(cmd.Password) < 8 {
+		return "", errors.ErrInvalidInput
+	}
+
+	existing, _ := h.userRepo.FindByEmail(ctx, cmd.Email)
+	if existing != nil {
+		return "", errors.ErrAlreadyExists
+	}
+
+	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(cmd.Password), bcrypt.DefaultCost)
+	if err != nil {
+		return "", err
+	}
+
+	timezone := cmd.Timezone
+	if timezone == "" {
+		timezone = "UTC"
+	}
+
+	user := entities.NewUser(cmd.Email, string(hashedPassword), timezone)
+
+	if err := h.userRepo.Create(ctx, user); err != nil {
+		return "", err
+	}
+
+	return user.ID, nil
+}