Add simplified mockup REST API specification for user LLM configuration with Bearer token authentication (#30)

* Initial plan

* Add comprehensive user management and LLM config API specification

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* Fix JSON examples and add proper document conclusion

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* Fix unused import and error message format consistency

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* Enhance reference section with descriptions and next steps

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* Simplify spec to mockup version with minimal code and config file backend

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* Address code review feedback: update Python version and clarify Docker network config

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* Add minimal authentication with fixed API key from config file

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* Fix code review issues: add os import, optimize config loading, improve tests

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* Update spec: Bearer auth, FastAPI, system_prompt, remove impl examples and future sections

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Author: Copilot