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- Remove immediate/ real-time module (superseded by IM service)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

@@ -0,0 +1,53 @@
+syntax = "proto3";
+
+package gitks;
+
+import "oid.proto";
+import "repository.proto";
+
+
+message FindRemoteRepositoryRequest {
+  string remote_url = 1;
+}
+
+message RemoteHead {
+  string ref_name = 1;
+  string target_oid = 2;
+  bool symbolic = 3;
+  string symbolic_target = 4;
+}
+
+message FindRemoteRepositoryResponse {
+  repeated RemoteHead refs = 1;
+  bool exists = 2;
+}
+
+message FindRemoteRootRefRequest {
+  string remote_url = 1;
+}
+
+message FindRemoteRootRefResponse {
+  string ref_name = 1;
+  string target_oid = 2;
+}
+
+
+message UpdateRemoteMirrorRequest {
+  RepositoryHeader repository = 1;
+  string remote_url = 2;
+  string remote_name = 3; // defaults to "origin"
+  bool force = 4;
+  bool prune = 5;
+  repeated string refspecs = 6; // if empty, fetch all refs
+}
+
+message UpdateRemoteMirrorResponse {
+  bool ok = 1;
+  string error = 2;
+}
+
+service RemoteService {
+  rpc FindRemoteRepository(FindRemoteRepositoryRequest) returns (FindRemoteRepositoryResponse);
+  rpc FindRemoteRootRef(FindRemoteRootRefRequest) returns (FindRemoteRootRefResponse);
+  rpc UpdateRemoteMirror(UpdateRemoteMirrorRequest) returns (UpdateRemoteMirrorResponse);
+}