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- Remove immediate/ real-time module (superseded by IM service)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/this/im/auth.proto

@@ -0,0 +1,26 @@
+syntax = "proto3";
+
+package appks.im.v1;
+
+// Internal service-to-service authentication.
+// appks issues API keys (stored in Redis), remote services
+// carry the key in gRPC metadata "x-api-key", and call
+// Authenticate to verify identity.
+
+message AuthenticateRequest {
+  string api_key = 1;
+}
+
+message AuthenticateResponse {
+  bool authenticated = 1;
+  string service_name = 2;
+  string service_id = 3;
+  repeated string scopes = 4;
+  int64 expires_at = 5;
+}
+
+service InternalAuthService {
+  // Verify an API key and return the associated service identity.
+  // Called by remote services to authenticate themselves.
+  rpc Authenticate(AuthenticateRequest) returns (AuthenticateResponse);
+}