feat(audio-cpp): AUDIOCPP_DEFAULT_BACKEND fallback for models without a backend option (#12133)

Models whose options carry no explicit backend: open their session on the
CPU backend even in accelerator images. The gallery entries carry
backend:best since #11892; this covers hand-written model configurations
the same way, per deployment: the environment variable supplies the
fallback, an explicit backend: option always wins (merged beside the
existing threads and maingpu fallbacks), and validation reuses the
option parser.

Assisted-by: Claude:claude-fable-5

Signed-off-by: Plamen K. Kosseff <p.kosseff@gmail.com>

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

@@ -596,6 +596,25 @@ public:
             if (!parsed.options.device_set && !request->maingpu().empty()) {
                 parsed.options.device = parse_device_index(request->maingpu());
             }
+            // AUDIOCPP_DEFAULT_BACKEND is the deployment's backend fallback:
+            // an accelerator image sets it (typically to "best") so models
+            // without an explicit backend: option use the compiled
+            // accelerator instead of the CPU default — gallery entries carry
+            // backend:best, hand-written model configs get the same fix
+            // here. An explicit backend: option wins, matching threads and
+            // maingpu above. Validation reuses the option parser itself.
+            if (!parsed.options.backend_set) {
+                const char * env = std::getenv("AUDIOCPP_DEFAULT_BACKEND");
+                if (env != nullptr && *env != '\0') {
+                    auto fallback = audiocpp_backend::parse_model_options(
+                        {std::string("backend:") + env});
+                    if (!fallback.error.empty()) {
+                        throw audiocpp_backend::ConfigError(
+                            "audio-cpp: AUDIOCPP_DEFAULT_BACKEND: " + fallback.error);
+                    }
+                    parsed.options.backend = fallback.options.backend;
+                }
+            }
 
             const std::string path = audiocpp_backend::resolve_model_path(
                 request->modelpath(), request->modelfile(), request->model());