feat: implement secure cluster authentication (issue #13)

Implemented a comprehensive secure authentication mechanism for inter-node
cluster communication with the following features:

1. Global Cluster Secret (GCS)
   - Auto-generated cryptographically secure random secret (256-bit)
   - Configurable via YAML config file
   - Shared across all cluster nodes for authentication

2. Cluster Authentication Middleware
   - Validates X-Cluster-Secret and X-Node-ID headers
   - Applied to all cluster endpoints (/members/*, /merkle_tree/*, /kv_range)
   - Comprehensive logging of authentication attempts

3. Authenticated HTTP Client
   - Custom HTTP client with cluster auth headers
   - TLS support with configurable certificate verification
   - Protocol-aware (http/https based on TLS settings)

4. Secure Bootstrap Endpoint
   - New /auth/cluster-bootstrap endpoint
   - Protected by JWT authentication with admin scope
   - Allows new nodes to securely obtain cluster secret

5. Updated Cluster Communication
   - All gossip protocol requests include auth headers
   - All Merkle tree sync requests include auth headers
   - All data replication requests include auth headers

6. Configuration
   - cluster_secret: Shared secret (auto-generated if not provided)
   - cluster_tls_enabled: Enable TLS for inter-node communication
   - cluster_tls_cert_file: Path to TLS certificate
   - cluster_tls_key_file: Path to TLS private key
   - cluster_tls_skip_verify: Skip TLS verification (testing only)

This implementation addresses the security vulnerability of unprotected
cluster endpoints and provides a flexible, secure approach to protecting
internal cluster communication while allowing for automated node bootstrapping.

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

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

cluster/http_client.go

@@ -0,0 +1,43 @@
+package cluster
+
+import (
+	"crypto/tls"
+	"net/http"
+	"time"
+
+	"kvs/types"
+)
+
+// NewAuthenticatedHTTPClient creates an HTTP client configured for cluster authentication
+func NewAuthenticatedHTTPClient(config *types.Config, timeout time.Duration) *http.Client {
+	client := &http.Client{
+		Timeout: timeout,
+	}
+
+	// Configure TLS if enabled
+	if config.ClusterTLSEnabled {
+		tlsConfig := &tls.Config{
+			InsecureSkipVerify: config.ClusterTLSSkipVerify,
+		}
+
+		client.Transport = &http.Transport{
+			TLSClientConfig: tlsConfig,
+		}
+	}
+
+	return client
+}
+
+// AddClusterAuthHeaders adds authentication headers to an HTTP request
+func AddClusterAuthHeaders(req *http.Request, config *types.Config) {
+	req.Header.Set("X-Cluster-Secret", config.ClusterSecret)
+	req.Header.Set("X-Node-ID", config.NodeID)
+}
+
+// GetProtocol returns the appropriate protocol (http or https) based on TLS configuration
+func GetProtocol(config *types.Config) string {
+	if config.ClusterTLSEnabled {
+		return "https"
+	}
+	return "http"
+}