backend(audio-cpp): add the native build scaffold

Links 0xShug0/audio.cpp engine_runtime through its public framework headers
and serves Health/Status. Model loading and the audio RPCs follow.

Assisted-by: Claude:claude-opus-5 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/cpp/audio-cpp/grpc-server.cpp

@@ -0,0 +1,102 @@
+// audio.cpp LocalAI gRPC backend.
+//
+// Links 0xShug0/audio.cpp's engine_runtime through its public
+// include/engine/framework/** headers only. Nothing under upstream's app/,
+// src/ or tests/ is used: those are application internals, they are where
+// upstream expects churn, and upstream is Apache-2.0 while LocalAI is MIT.
+//
+// This commit is the bind/listen/Health/Status skeleton. Model loading and the
+// audio RPCs land in later commits.
+
+#include "backend.pb.h"
+#include "backend.grpc.pb.h"
+
+#include <grpcpp/grpcpp.h>
+#include <grpcpp/server.h>
+#include <grpcpp/server_builder.h>
+#include <grpcpp/ext/proto_server_reflection_plugin.h>
+
+#include <atomic>
+#include <chrono>
+#include <csignal>
+#include <cstdlib>
+#include <iostream>
+#include <memory>
+#include <string>
+
+using grpc::Server;
+using grpc::ServerBuilder;
+using grpc::ServerContext;
+// Do NOT alias grpc::Status as Status: the Status RPC method would shadow the
+// type and break every other method that names it as a return type.
+using GStatus = ::grpc::Status;
+
+namespace {
+
+std::atomic<Server *> g_server{nullptr};
+
+class AudioCppBackend final : public backend::Backend::Service {
+public:
+    GStatus Health(ServerContext *, const backend::HealthMessage *,
+                   backend::Reply *reply) override {
+        reply->set_message("OK");
+        return GStatus::OK;
+    }
+
+    GStatus Status(ServerContext *, const backend::HealthMessage *,
+                   backend::StatusResponse *response) override {
+        response->set_state(backend::StatusResponse::UNINITIALIZED);
+        return GStatus::OK;
+    }
+};
+
+void RunServer(const std::string &addr) {
+    AudioCppBackend service;
+    grpc::EnableDefaultHealthCheckService(true);
+    grpc::reflection::InitProtoReflectionServerBuilderPlugin();
+
+    ServerBuilder builder;
+    builder.AddListeningPort(addr, grpc::InsecureServerCredentials());
+    builder.RegisterService(&service);
+    // Audio payloads (PCM buffers, encoded frames) are far larger than the
+    // 4 MiB gRPC default.
+    builder.SetMaxReceiveMessageSize(256 * 1024 * 1024);
+    builder.SetMaxSendMessageSize(256 * 1024 * 1024);
+
+    std::unique_ptr<Server> server(builder.BuildAndStart());
+    if (!server) {
+        std::cerr << "audio-cpp grpc-server: failed to bind " << addr << "\n";
+        std::exit(1);
+    }
+    g_server = server.get();
+    std::cerr << "audio-cpp grpc-server listening on " << addr << "\n";
+    server->Wait();
+}
+
+void signal_handler(int) {
+    if (auto *srv = g_server.load()) {
+        srv->Shutdown(std::chrono::system_clock::now() + std::chrono::seconds(3));
+    }
+}
+
+} // namespace
+
+int main(int argc, char *argv[]) {
+    std::string addr = "127.0.0.1:50051";
+    for (int i = 1; i < argc; ++i) {
+        std::string a = argv[i];
+        const std::string addr_flag = "--addr=";
+        if (a.rfind(addr_flag, 0) == 0) {
+            addr = a.substr(addr_flag.size());
+        } else if (a == "--addr" && i + 1 < argc) {
+            addr = argv[++i];
+        } else if (a == "--help" || a == "-h") {
+            std::cout << "Usage: grpc-server --addr=HOST:PORT\n";
+            return 0;
+        }
+    }
+    std::signal(SIGINT, signal_handler);
+    std::signal(SIGTERM, signal_handler);
+    RunServer(addr);
+    return 0;
+}