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/infrastructure/auth/jwt.go

@@ -0,0 +1,59 @@
+package auth
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID string `json:"user_id"`
+	Email  string `json:"email"`
+	jwt.RegisteredClaims
+}
+
+type JWTService struct {
+	secret     []byte
+	expiry     time.Duration
+}
+
+func NewJWTService(secret string, expiryHours int) *JWTService {
+	return &JWTService{
+		secret:     []byte(secret),
+		expiry:     time.Duration(expiryHours) * time.Hour,
+	}
+}
+
+func (s *JWTService) GenerateToken(userID, email string) (string, error) {
+	claims := Claims{
+		UserID: userID,
+		Email:  email,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(s.expiry)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(s.secret)
+}
+
+func (s *JWTService) ValidateToken(tokenString string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenString, &Claims{}, func(token *jwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
+		}
+		return s.secret, nil
+	})
+
+	if err != nil {
+		return nil, err
+	}
+
+	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		return claims, nil
+	}
+
+	return nil, fmt.Errorf("invalid token")
+}