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/queries/login_user.go

@@ -0,0 +1,50 @@
+package queries
+
+import (
+	"context"
+
+	"habit-tracker-api/internal/domain/repositories"
+	"habit-tracker-api/internal/shared/errors"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+type LoginUserQuery struct {
+	Email    string
+	Password string
+}
+
+type LoginUserResult struct {
+	UserID   string
+	Email    string
+	Timezone string
+}
+
+type LoginUserHandler struct {
+	userRepo repositories.UserRepository
+}
+
+func NewLoginUserHandler(userRepo repositories.UserRepository) *LoginUserHandler {
+	return &LoginUserHandler{userRepo: userRepo}
+}
+
+func (h *LoginUserHandler) Handle(ctx context.Context, query LoginUserQuery) (*LoginUserResult, error) {
+	if query.Email == "" || query.Password == "" {
+		return nil, errors.ErrInvalidInput
+	}
+
+	user, err := h.userRepo.FindByEmail(ctx, query.Email)
+	if err != nil {
+		return nil, errors.ErrNotFound
+	}
+
+	if err := bcrypt.CompareHashAndPassword([]byte(user.PasswordHash), []byte(query.Password)); err != nil {
+		return nil, errors.ErrNotFound
+	}
+
+	return &LoginUserResult{
+		UserID:   user.ID,
+		Email:    user.Email,
+		Timezone: user.Timezone,
+	}, nil
+}