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- Remove immediate/ real-time module (superseded by IM service)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

etcd/types.rs

@@ -8,3 +8,19 @@ pub struct ServiceInstance {
     #[serde(default)]
     pub metadata: HashMap<String, String>,
 }
+
+/// Information about a gitks peer node, registered in etcd under /gitks/nodes/.
+/// Mirrors gitks::cluster::types::PeerInfo.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct GitksPeerInfo {
+    /// Logical storage name (e.g. "node-a", "default")
+    pub storage_name: String,
+    /// ractor_cluster TCP address (e.g. "10.0.1.4:4697")
+    #[serde(default)]
+    pub cluster_addr: String,
+    /// gRPC service address (e.g. "http://10.0.1.4:50051")
+    pub grpc_addr: String,
+    /// Software version
+    #[serde(default)]
+    pub version: String,
+}