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>

features/validation.go

@@ -21,4 +21,4 @@ func ParseTTL(ttlString string) (time.Duration, error) {
 	}
 
 	return duration, nil
-}
+}