refactor(backend): 启动环境初始化与资源项目化（零跨项目依赖）

- main.py：uv run main.py 一键启动——读取 backend/.env、TORCH_HOME/NLTK_DATA 指向项目内缓存、certifi SSL + NO_PROXY 网络直连、默认开启 s2s，启动横幅
- vendor/：补丁后 s2s 云化栈移入项目（backend/vendor/s2s-cloud），pyproject sources 改相对路径
- .gitignore：忽略 .torch-cache / nltk_data 缓存
- AGENTS.md：进程管理铁律（AI 不启动/重启服务，由用户操作）

backend/vendor/s2s-cloud/tests/openai_realtime/conftest.py

@@ -0,0 +1,60 @@
+from queue import Queue
+from threading import Event as ThreadingEvent
+
+import pytest
+from openai.types.realtime import RealtimeSessionCreateRequest
+from openai.types.realtime.realtime_audio_config import RealtimeAudioConfig
+from openai.types.realtime.realtime_audio_config_input import RealtimeAudioConfigInput
+from openai.types.realtime.realtime_audio_config_output import RealtimeAudioConfigOutput
+from openai.types.realtime.realtime_audio_formats import AudioPCM
+
+from speech_to_speech.api.openai_realtime.runtime_config import RuntimeConfig
+from speech_to_speech.api.openai_realtime.service import RealtimeService
+
+
+def _session_16k() -> RealtimeSessionCreateRequest:
+    """Build a test session with 16 kHz audio rates (matches PIPELINE_SAMPLE_RATE)."""
+    fmt = AudioPCM.model_construct(rate=16000, type="audio/pcm")
+    return RealtimeSessionCreateRequest.model_construct(
+        type="realtime",
+        audio=RealtimeAudioConfig.model_construct(
+            input=RealtimeAudioConfigInput.model_construct(format=fmt),
+            output=RealtimeAudioConfigOutput.model_construct(format=fmt),
+        ),
+    )
+
+
+@pytest.fixture
+def runtime_config():
+    cfg = RuntimeConfig()
+    cfg.session = _session_16k()
+    return cfg
+
+
+@pytest.fixture
+def text_prompt_queue():
+    return Queue()
+
+
+@pytest.fixture
+def should_listen():
+    ev = ThreadingEvent()
+    ev.set()
+    return ev
+
+
+@pytest.fixture
+def service(runtime_config, text_prompt_queue, should_listen):
+    svc = RealtimeService(
+        text_prompt_queue=text_prompt_queue,
+        should_listen=should_listen,
+    )
+    return svc
+
+
+@pytest.fixture
+def conn_id(service, runtime_config):
+    cid = service.register()
+    service._state(cid).runtime_config = runtime_config
+    yield cid
+    service.unregister(cid)