refactor: extract authentication system to auth package

- Create auth/jwt.go with JWT token management
- Create auth/permissions.go with permission checking logic
- Create auth/storage.go with storage key utilities
- Create auth/auth.go with main authentication service
- Create auth/middleware.go with auth and rate limit middleware
- Update main.go to import auth package and use auth.* functions
- Add authService to Server struct

Major auth functionality now separated into dedicated package.
Build tested and verified working.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

auth/jwt.go

@@ -0,0 +1,67 @@
+package auth
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v4"
+)
+
+// JWT signing key (should be configurable in production)
+var jwtSigningKey = []byte("your-secret-signing-key-change-this-in-production")
+
+// JWTClaims represents the custom claims for our JWT tokens
+type JWTClaims struct {
+	UserUUID string   `json:"user_uuid"`
+	Scopes   []string `json:"scopes"`
+	jwt.RegisteredClaims
+}
+
+// GenerateJWT creates a new JWT token for a user with specified scopes
+func GenerateJWT(userUUID string, scopes []string, expirationHours int) (string, int64, error) {
+	if expirationHours <= 0 {
+		expirationHours = 1 // Default to 1 hour
+	}
+
+	now := time.Now()
+	expiresAt := now.Add(time.Duration(expirationHours) * time.Hour)
+
+	claims := JWTClaims{
+		UserUUID: userUUID,
+		Scopes:   scopes,
+		RegisteredClaims: jwt.RegisteredClaims{
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(expiresAt),
+			Issuer:    "kvs-server",
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	tokenString, err := token.SignedString(jwtSigningKey)
+	if err != nil {
+		return "", 0, err
+	}
+
+	return tokenString, expiresAt.Unix(), nil
+}
+
+// ValidateJWT validates a JWT token and returns the claims if valid
+func ValidateJWT(tokenString string) (*JWTClaims, error) {
+	token, err := jwt.ParseWithClaims(tokenString, &JWTClaims{}, func(token *jwt.Token) (interface{}, error) {
+		// Validate signing method
+		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
+		}
+		return jwtSigningKey, nil
+	})
+
+	if err != nil {
+		return nil, err
+	}
+
+	if claims, ok := token.Claims.(*JWTClaims); ok && token.Valid {
+		return claims, nil
+	}
+
+	return nil, fmt.Errorf("invalid token")
+}