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/test_transcription_notifier.py

@@ -0,0 +1,80 @@
+import logging
+from queue import Queue
+from threading import Event
+
+from speech_to_speech.api.openai_realtime.runtime_config import RuntimeConfig
+from speech_to_speech.pipeline.events import PartialTranscriptionEvent, TranscriptionCompletedEvent
+from speech_to_speech.pipeline.messages import GenerateResponseRequest, PartialTranscription, Transcription
+from speech_to_speech.STT.transcription_notifier import TranscriptionNotifier
+
+
+def _notifier(
+    text_output_queue: Queue | None = None,
+    runtime_config: RuntimeConfig | None = None,
+    should_listen: Event | None = None,
+) -> TranscriptionNotifier:
+    notifier = object.__new__(TranscriptionNotifier)
+    notifier.setup(text_output_queue=text_output_queue, runtime_config=runtime_config, should_listen=should_listen)
+    return notifier
+
+
+def test_empty_final_transcription_still_emits_completion_after_partial():
+    text_output_queue = Queue()
+    notifier = _notifier(text_output_queue=text_output_queue)
+
+    assert list(notifier.process(PartialTranscription(text="Yeah."))) == []
+    assert list(notifier.process(Transcription(text="", language_code="en", speech_stopped_at_s=123.0))) == []
+
+    partial = text_output_queue.get_nowait()
+    completed = text_output_queue.get_nowait()
+
+    assert isinstance(partial, PartialTranscriptionEvent)
+    assert partial.delta == "Yeah."
+    assert isinstance(completed, TranscriptionCompletedEvent)
+    assert completed.transcript == ""
+    assert completed.language_code == "en"
+    assert completed.speech_stopped_at_s == 123.0
+    assert text_output_queue.empty()
+
+
+def test_empty_final_transcription_does_not_trigger_legacy_generation():
+    runtime_config = RuntimeConfig()
+    should_listen = Event()
+    notifier = _notifier(runtime_config=runtime_config, should_listen=should_listen)
+
+    assert list(notifier.process(Transcription(text="", language_code="en"))) == []
+    assert should_listen.is_set()
+
+
+def test_non_empty_final_transcription_still_triggers_legacy_generation():
+    runtime_config = RuntimeConfig()
+    should_listen = Event()
+    notifier = _notifier(runtime_config=runtime_config, should_listen=should_listen)
+
+    result = list(notifier.process(Transcription(text="hello", language_code="en", speech_stopped_at_s=123.0)))
+
+    assert len(result) == 1
+    assert isinstance(result[0], GenerateResponseRequest)
+    assert result[0].runtime_config is runtime_config
+    assert result[0].language_code == "en"
+    assert result[0].speech_stopped_at_s == 123.0
+    assert not should_listen.is_set()
+
+
+def test_non_empty_final_transcription_logs_full_text_at_info(caplog):
+    notifier = _notifier()
+    transcript = "hello " * 30
+
+    with caplog.at_level(logging.INFO, logger="speech_to_speech.STT.transcription_notifier"):
+        assert list(notifier.process(Transcription(text=transcript, language_code="en"))) == []
+
+    assert "Transcription completed (language=en): " + transcript in caplog.text
+
+
+def test_empty_final_transcription_reenables_listening_without_runtime_config():
+    should_listen = Event()
+    notifier = _notifier(should_listen=should_listen)
+
+    assert list(notifier.process(Transcription(text="", language_code="en"))) == []
+
+    assert should_listen.is_set()