feat: add gateway module with FastAPI server (#5)

* chore: add .claude/ to .gitignore

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat: add gateway module with FastAPI server

- Add new gateway module with FastAPI app for API routing
- Add gateway and serve commands to Makefile
- Add fastapi, httpx, uvicorn, sse-starlette dependencies
- Fix model config retrieval in lead_agent (support both model_name and model)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -23,4 +23,5 @@ config.yaml
 # Coverage report
 coverage.xml
 coverage/
-.deer-flow/
+.deer-flow/
+.claude/