add litellm integration plan

TODO.md

@@ -0,0 +1,56 @@
+# LiteLLM Rust integration plan
+
+Tinychat already supports the required integration model: it sends streaming
+`POST /v1/chat/completions` requests to any `OPENAI_BASE_URL`. Therefore,
+integrate LiteLLM Rust as an external gateway rather than adding an unstable
+SDK or crate to the binary.
+
+> **Status note:** LiteLLM's official Rust migration is still in progress. The
+> `/chat/completions` route is planned after the `/messages` migration, with a
+> stated target of September 1, 2026. Do not make the proof-of-concept
+> `LiteLLM-Labs/litellm-rust` repository a production dependency.
+
+## Plan
+
+- [ ] Define the MVP: support only streaming Chat Completions; exclude the
+  Responses API, tool calls, and provider-specific parameters. Preserve the
+  current OpenAI-compatible path as the fallback.
+- [ ] Obtain the official Rust beta, once its `/v1/chat/completions` endpoint
+  is available, and pin an exact image tag or revision. Verify its license and
+  minimum supported Rust version.
+- [ ] Prepare a minimal `litellm_config.yaml` containing model routes, keys
+  supplied through environment variables, and a master key. Never commit
+  secrets.
+- [ ] Document gateway startup and Tinychat configuration in `README.md`:
+
+  ```sh
+  OPENAI_BASE_URL=http://127.0.0.1:4000/v1
+  OPENAI_API_KEY=$LITELLM_MASTER_KEY
+  OPENAI_MODEL=<route-name>
+  ```
+
+  Keep the existing CLI flags and environment-variable interface unchanged.
+
+- [ ] Add contract tests against a local mock gateway covering the request URL,
+  `Authorization` header, payload, SSE deltas, usage, `[DONE]`, 4xx/5xx
+  errors, timeouts, cancellation, and existing response-size limits.
+- [ ] Run an integration smoke test with the Rust gateway and one provider,
+  without exposing credentials: normal streaming, response cancellation,
+  invalid key, and unavailable model route.
+- [ ] Add an opt-in CI job (manual or scheduled) that exercises the mock or a
+  local gateway. Restrict live-provider checks to protected environments.
+- [ ] Roll out in stages: experimental documentation, beta on a pinned
+  gateway version, then a default recommendation after several compatible
+  releases without regressions. Rollback remains a one-variable change to
+  `OPENAI_BASE_URL`.
+
+## Done criteria
+
+Tinychat works through the official LiteLLM Rust gateway without UI or API
+changes and retains streaming, SOCKS5 proxy support, timeouts, cancellation,
+and bounded SSE response handling.
+
+## References
+
+- [LiteLLM Rust migration roadmap](https://docs.litellm.ai/blog/litellm-rust-launch)
+- [LiteLLM Rust migration tracking issue](https://github.com/BerriAI/litellm/issues/31263)