update frontend references to working Vercel URL

phase-1: CI workflow, ruff linting, keepalive alerting, VITE_API_URL, pre-commit hooks

feat: complete 4-phase improvement strategy

checkpoint: pre-PNP-refactor state

fix: hardcode API URL to HF Space

feat: add HF Space deployment config, update frontend API URL

phase2+3: backend API + Astro frontend