chore(infra): add gRPC layer, update protobufs, remove immediate module

- Add gRPC service modules: auth, channel, channel settings, member,
  permission
- Update protobuf definitions and generated code
- Remove immediate/ real-time module (superseded by IM service)
- Update etcd discovery and registration
- Update cache, error, config, and build infrastructure
- Add ADR documentation
- Update OpenAPI spec

proto/git/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;
+}