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/demo/CONTEXT.md

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+# Context glossary
+
+Canonical terms for this space. A glossary, not a spec — it defines what words mean,
+not how anything is built. Keep design/implementation detail in `DESIGN.md` and the
+code.
+
+## Speech-to-speech demo
+The product: a voice conversation you have with a model by tapping the orb and
+talking. "The demo" and "the space" refer to this same thing. It runs on Hugging
+Face's open `speech-to-speech` backend.
+
+## The pipeline
+The ordered path a turn travels, from your voice to the orb's reply. Order is
+meaningful — each stage consumes the previous one's output:
+
+`you speak → VAD → STT → VLM → TTS → orb replies`
+
+- **VAD** — voice activity detection. Decides *when* you are speaking, so the system
+  knows a turn has started and ended. Model: silero-vad.
+- **STT** — speech to text. Transcribes your speech into words. Model:
+  nvidia/parakeet-tdt-1.1b.
+- **VLM** — the vision-language model that composes the reply. Served via Cerebras.
+  Model: google/gemma-4-31B-it.
+- **TTS** — text to speech. Speaks the reply back in the chosen voice. Model:
+  Qwen/Qwen3-TTS-12Hz-1.7B-CustomVoice.
+
+## Builder
+A Hugging Face user credited with making the space, shown by HF username. Current
+builders: tfrere, A-Mahla and andito. Distinct from the *models'* authors (nvidia, google,
+Qwen, snakers4), who are credited per pipeline stage.
+
+## Powered by
+The infrastructure running the pipeline, named in the about panel: Hugging Face
+Inference Endpoints (hosting) and Cerebras (LLM inference). Distinct from "built by"
+(the people) and from the model authors (who trained each model).
+
+## Tool
+A function the model can call mid-conversation to do something the pipeline
+can't do on its own (look something up, look through the camera). Tools are
+declared to the backend in the session config; the model decides when to call
+one. Distinct from the *pipeline stages* (VAD/STT/VLM/TTS), which always run.
+
+## Tool executor
+The client-side component that runs a tool when the model calls it and returns
+the result to the backend, so the model can speak the answer. It is the missing
+half of the round-trip: the backend already emits the call, the executor runs it
+and replies. Distinct from the *tool* itself (the thing being run).
+
+## Web search tool
+A tool that looks something up on the web for the model. The model calls it with
+a query; the tool executor forwards the query to the search proxy and returns the
+results as the tool result. Activates only when a search key is available.
+
+## Camera snapshot tool
+A tool that lets the model look through the user's webcam. While enabled, a live
+self-view is shown in the page (bottom-left); when the model calls the tool, the
+executor captures a frame and sends it to the model as an image so the VLM can
+see it. Distinct from the *preview* (what the user sees) and the *snapshot* (the
+single frame sent to the model).
+
+## Search proxy
+The same-origin server route (`/search`) that holds the search key and calls the
+external search provider on the client's behalf, so the key never reaches the
+browser. Lives in the same container as the page. Distinct from the *s2s backend*
+(the separate load-balanced speech-to-speech service).
+
+## Tools panel
+The dialog opened from the "Tools" button in the top-right, holding one switch per
+tool (and the web-search key status). Turning a switch on/off declares or removes
+that tool on the live session. Distinct from *Settings* (connection, voice,
+instructions) and the *About panel* (project info).
+
+## Identity block
+The top-left corner of the topbar (replacing the old wordmark): the demo name, a
+one-line blurb, and the "powered by" / "built by" credits, shown directly rather
+than hidden behind a click.
+
+## About panel
+The popup opened from the (i) icon to the right of the identity block. Holds the
+general introduction to the speech-to-speech project (with a repo link) and the
+pipeline. Identity itself now lives in the corner, not here.
+
+## Queue
+The line of users waiting for a free conversation slot when every compute is
+busy. You join the queue instead of being turned away; you leave it by reaching
+the front or by giving up. Distinct from a *session* (an actual live
+conversation) — being in the queue is not yet talking, and time spent waiting
+never counts against your usage limit.
+
+## Ticket
+Your held place in the queue. Created when you join, it is what the demo checks
+to tell you your position and to notice if you have left. A ticket is not a
+session: it only promises a spot in line, not a compute.
+
+## Position
+How many people are ahead of you in the queue, shown while you wait ("You're #3
+in line"). It only ever counts down. Distinct from an *estimated wait* — the
+demo shows position, never a time, because wait time is unpredictable.
+
+## Claim
+The moment you reach the front and a free slot becomes yours — the queue hands
+off to a real session and the conversation begins. This is also the point where
+your usage limit first starts to matter (never while waiting).
+
+## At capacity
+The state where the queue itself is full, so new users can't even join the line
+and are asked to try again shortly. Distinct from simply *busy* (all computes
+taken but the queue still has room to wait in).