feat(cluster): implement distributed clustering with etcd coordination

- Integrate etcd-client for distributed coordination and leader election
- Add remote client macros with proper formatting for all services
- Implement RequestMetrics for tracking RPC performance and errors
- Add rate limiting mechanism across all service endpoints
- Create ElectionRequest and ElectionResult message types for leader election
- Add role management with primary/replica switching capabilities
- Implement health checker with automatic failover detection
- Add repository count metrics for cluster monitoring
- Update Cargo.toml with etcd-client and dashmap dependencies
- Modify RepoEntry to include read_only flag for replica handling
- Implement should_accept_election logic to prevent duplicate elections
- Add RoleChangedEvent handling for cluster role updates

cluster/types.rs

@@ -0,0 +1,14 @@
+use serde::{Deserialize, Serialize};
+
+/// Information about a peer node, registered in etcd.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PeerInfo {
+    /// Logical storage name (e.g. "node-a", "default")
+    pub storage_name: String,
+    /// ractor_cluster TCP address (e.g. "10.0.1.4:4697")
+    pub cluster_addr: String,
+    /// gRPC service address (e.g. "http://10.0.1.4:50051")
+    pub grpc_addr: String,
+    /// Software version
+    pub version: String,
+}