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>

storage/storage.go

@@ -12,17 +12,17 @@ import (
 
 // StorageService handles all BadgerDB operations and data management
 type StorageService struct {
-	db               *badger.DB
-	config           *types.Config
-	compressionSvc   *CompressionService
-	logger           *logrus.Logger
+	db             *badger.DB
+	config         *types.Config
+	compressionSvc *CompressionService
+	logger         *logrus.Logger
 }
 
 // NewStorageService creates a new storage service
 func NewStorageService(db *badger.DB, config *types.Config, logger *logrus.Logger) (*StorageService, error) {
 	var compressionSvc *CompressionService
 	var err error
-	
+
 	// Initialize compression if enabled
 	if config.CompressionEnabled {
 		compressionSvc, err = NewCompressionService()
@@ -50,7 +50,7 @@ func (s *StorageService) Close() {
 func (s *StorageService) StoreWithTTL(txn *badger.Txn, key []byte, data []byte, ttl time.Duration) error {
 	var finalData []byte
 	var err error
-	
+
 	// Compress data if compression is enabled
 	if s.config.CompressionEnabled && s.compressionSvc != nil {
 		finalData, err = s.compressionSvc.CompressData(data)
@@ -60,14 +60,14 @@ func (s *StorageService) StoreWithTTL(txn *badger.Txn, key []byte, data []byte,
 	} else {
 		finalData = data
 	}
-	
+
 	entry := badger.NewEntry(key, finalData)
-	
+
 	// Apply TTL if specified
 	if ttl > 0 {
 		entry = entry.WithTTL(ttl)
 	}
-	
+
 	return txn.SetEntry(entry)
 }
 
@@ -77,7 +77,7 @@ func (s *StorageService) RetrieveWithDecompression(txn *badger.Txn, key []byte)
 	if err != nil {
 		return nil, err
 	}
-	
+
 	var compressedData []byte
 	err = item.Value(func(val []byte) error {
 		compressedData = append(compressedData, val...)
@@ -86,12 +86,12 @@ func (s *StorageService) RetrieveWithDecompression(txn *badger.Txn, key []byte)
 	if err != nil {
 		return nil, err
 	}
-	
+
 	// Decompress data if compression is enabled
 	if s.config.CompressionEnabled && s.compressionSvc != nil {
 		return s.compressionSvc.DecompressData(compressedData)
 	}
-	
+
 	return compressedData, nil
 }
 
@@ -109,4 +109,4 @@ func (s *StorageService) DecompressData(compressedData []byte) ([]byte, error) {
 		return compressedData, nil
 	}
 	return s.compressionSvc.DecompressData(compressedData)
-}
+}