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

Cargo.lock

@@ -457,6 +457,24 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "etcd-client"
+version = "0.18.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5ed900ba953ca6bf1fadb75e0c6b73d8463b9e2bb6bdb7b4573e8e7295852fbe"
+dependencies = [
+ "http",
+ "prost",
+ "tokio",
+ "tokio-stream",
+ "tonic",
+ "tonic-build",
+ "tonic-prost",
+ "tonic-prost-build",
+ "tower",
+ "tower-service",
+]
+
 [[package]]
 name = "faster-hex"
 version = "0.10.0"
@@ -668,8 +686,10 @@ name = "gitks"
 version = "1.0.0"
 dependencies = [
  "async-trait",
+ "dashmap",
  "dotenvy",
  "duct",
+ "etcd-client",
  "gix",
  "gix-archive",
  "moka",
@@ -678,6 +698,8 @@ dependencies = [
  "ractor",
  "ractor_cluster",
  "serde",
+ "serde_json",
+ "sha2",
  "tempfile",
  "thiserror",
  "tokio",
@@ -2462,6 +2484,7 @@ dependencies = [
  "aws-lc-rs",
  "log",
  "once_cell",
+ "ring",
  "rustls-pki-types",
  "rustls-webpki",
  "subtle",
@@ -2936,6 +2959,7 @@ dependencies = [
  "socket2",
  "sync_wrapper",
  "tokio",
+ "tokio-rustls",
  "tokio-stream",
  "tower",
  "tower-layer",