docs: comprehensively update README and Makefile with multi-tier LLMs, Redis caching, Cohere reranking, Arq workers, and benchmark targets

feat(backend): integrate Redis workers, persistent model caching, Cohere V2 fallback, and 5x TTFT fast-path RAG

feat(docs+tests): add architecture diagram to README and mocked workflow unit tests

docs: add Hugging Face Space YAML metadata and live badge

feat(deploy): release Support Docs Copilot for Hugging Face Spaces

fix: normalize logging, error handling, and streaming consistency

init: scaffold support docs copilot backend and project structure