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- Remove immediate/ real-time module (superseded by IM service)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

@@ -10,6 +10,7 @@ message GitProtocolFeatures {
   repeated string capabilities = 2;
   repeated string server_options = 3;
   repeated string agent = 4;
+  bool stateless = 5;
 }
 
 message ReferenceAdvertisement {
@@ -24,11 +25,13 @@ message AdvertiseRefsRequest {
   RepositoryHeader repository = 1;
   GitProtocolFeatures protocol = 2;
   string service = 3;
+  bool raw = 4;
 }
 
 message AdvertiseRefsResponse {
   repeated ReferenceAdvertisement references = 1;
   repeated string capabilities = 2;
+  bytes raw_data = 3;
 }
 
 message UploadPackRequest {