feat: Implement JWT Utilities (Issue #2)

- Create JWTManager for token operations
- Implement GenerateAccessToken with user claims
- Implement GenerateRefreshToken
- Implement ValidateToken with expiration checks
- Implement ValidateRefreshToken
- Add ExtractTokenFromHeader helper
- Add comprehensive error handling

Closes #2

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/utils/jwt.go

@@ -0,0 +1,151 @@
+package utils
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/yourusername/victorialogs-manager/internal/models"
+)
+
+// Claims represents the JWT claims structure
+type Claims struct {
+	UserID      string              `json:"user_id"`
+	Username    string              `json:"username"`
+	Role        models.Role         `json:"role"`
+	Permissions []models.Permission `json:"permissions"`
+	jwt.RegisteredClaims
+}
+
+// JWTManager handles JWT token operations
+type JWTManager struct {
+	secretKey             string
+	accessTokenDuration   time.Duration
+	refreshTokenDuration  time.Duration
+}
+
+// NewJWTManager creates a new JWT manager
+func NewJWTManager(secretKey string, accessDuration, refreshDuration time.Duration) *JWTManager {
+	return &JWTManager{
+		secretKey:            secretKey,
+		accessTokenDuration:  accessDuration,
+		refreshTokenDuration: refreshDuration,
+	}
+}
+
+// GenerateAccessToken generates a new JWT access token for a user
+func (m *JWTManager) GenerateAccessToken(user *models.User) (string, error) {
+	now := time.Now()
+	expiresAt := now.Add(m.accessTokenDuration)
+
+	claims := Claims{
+		UserID:      user.ID,
+		Username:    user.Username,
+		Role:        user.Role,
+		Permissions: user.GetPermissions(),
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(expiresAt),
+			IssuedAt:  jwt.NewNumericDate(now),
+			NotBefore: jwt.NewNumericDate(now),
+			Issuer:    "victorialogs-manager",
+			Subject:   user.ID,
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(m.secretKey))
+}
+
+// GenerateRefreshToken generates a new JWT refresh token
+func (m *JWTManager) GenerateRefreshToken(userID string) (string, error) {
+	now := time.Now()
+	expiresAt := now.Add(m.refreshTokenDuration)
+
+	claims := jwt.RegisteredClaims{
+		ExpiresAt: jwt.NewNumericDate(expiresAt),
+		IssuedAt:  jwt.NewNumericDate(now),
+		NotBefore: jwt.NewNumericDate(now),
+		Issuer:    "victorialogs-manager",
+		Subject:   userID,
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString([]byte(m.secretKey))
+}
+
+// ValidateToken validates and parses a JWT token
+func (m *JWTManager) ValidateToken(tokenString string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(
+		tokenString,
+		&Claims{},
+		func(token *jwt.Token) (interface{}, error) {
+			// Verify signing method
+			if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+				return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
+			}
+			return []byte(m.secretKey), nil
+		},
+	)
+
+	if err != nil {
+		return nil, fmt.Errorf("invalid token: %w", err)
+	}
+
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, fmt.Errorf("invalid token claims")
+	}
+
+	// Check if token is expired
+	if claims.ExpiresAt != nil && claims.ExpiresAt.Time.Before(time.Now()) {
+		return nil, fmt.Errorf("token has expired")
+	}
+
+	return claims, nil
+}
+
+// ValidateRefreshToken validates a refresh token and returns the user ID
+func (m *JWTManager) ValidateRefreshToken(tokenString string) (string, error) {
+	token, err := jwt.ParseWithClaims(
+		tokenString,
+		&jwt.RegisteredClaims{},
+		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 []byte(m.secretKey), nil
+		},
+	)
+
+	if err != nil {
+		return "", fmt.Errorf("invalid refresh token: %w", err)
+	}
+
+	claims, ok := token.Claims.(*jwt.RegisteredClaims)
+	if !ok || !token.Valid {
+		return "", fmt.Errorf("invalid refresh token claims")
+	}
+
+	// Check if token is expired
+	if claims.ExpiresAt != nil && claims.ExpiresAt.Time.Before(time.Now()) {
+		return "", fmt.Errorf("refresh token has expired")
+	}
+
+	return claims.Subject, nil
+}
+
+// ExtractTokenFromHeader extracts JWT token from Authorization header
+// Expected format: "Bearer <token>"
+func ExtractTokenFromHeader(authHeader string) (string, error) {
+	const bearerPrefix = "Bearer "
+
+	if len(authHeader) < len(bearerPrefix) {
+		return "", fmt.Errorf("invalid authorization header format")
+	}
+
+	if authHeader[:len(bearerPrefix)] != bearerPrefix {
+		return "", fmt.Errorf("authorization header must start with 'Bearer '")
+	}
+
+	return authHeader[len(bearerPrefix):], nil
+}