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

proto/hooks.proto

@@ -0,0 +1,61 @@
+syntax = "proto3";
+
+package gitks;
+
+import "repository.proto";
+
+// HookService provides gRPC callback hooks for git operations.
+// External services can implement this interface to receive hook callbacks.
+service HookService {
+  // Pre-receive hook callback: validate push before it happens.
+  rpc PreReceiveHook(PreReceiveHookRequest) returns (PreReceiveHookResponse);
+
+  // Update hook callback: validate each ref update individually.
+  rpc UpdateHook(UpdateHookRequest) returns (UpdateHookResponse);
+
+  // Post-receive hook callback: notify after push has completed.
+  rpc PostReceiveHook(PostReceiveHookRequest) returns (PostReceiveHookResponse);
+}
+
+message PreReceiveHookRequest {
+  RepositoryHeader repository = 1;
+  repeated RefUpdate ref_updates = 2;
+  string push_options = 3;
+}
+
+message PreReceiveHookResponse {
+  bool accept = 1;
+  string rejection_message = 2;
+}
+
+message UpdateHookRequest {
+  RepositoryHeader repository = 1;
+  string ref_name = 2;
+  string old_oid = 3;
+  string new_oid = 4;
+}
+
+message UpdateHookResponse {
+  bool accept = 1;
+  string rejection_message = 2;
+}
+
+message PostReceiveHookRequest {
+  RepositoryHeader repository = 1;
+  repeated RefUpdate ref_updates = 2;
+}
+
+message PostReceiveHookResponse {
+  repeated HookAction actions = 1;
+}
+
+message RefUpdate {
+  string old_oid = 1;
+  string new_oid = 2;
+  string ref_name = 3;
+}
+
+message HookAction {
+  string action_type = 1;  // "trigger_ci", "update_index", etc.
+  string payload = 2;
+}