fix: add xlrd for .xls support, use explicit engine in read_excel

Add health endpoint and cold-start wake guidance

feat: download Qwen2.5-Coder-0.5B + BART at build, add few-shot prompts

fix: remove runtime model dependency and repair chart generation

fix: make model loading offline-safe for Spaces runtime

config: add HF Spaces YAML configuration to README

deploy: production backend code for HF Space