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- Remove immediate/ real-time module (superseded by IM service)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

pb/im.rs

@@ -0,0 +1,5 @@
+// Generated from proto/this/im/*.proto (package appks.im.v1)
+// Compiled via tonic-build in build.rs using OUT_DIR + include!
+// Build server = true, build client = false (appks is the server side for IM RPCs).
+
+include!(concat!(env!("OUT_DIR"), "/appks.im.v1.rs"));