fix(ci): drop py3.12 matrix entry and stop tracking pycache under src/captioning/data

fix(ci): set MYPYPATH env and explicit-package-bases for mypy step

fix(ci): use list-form mypy_path and switch frontend job to npm install

feat(ci): add deploy-backend workflow + Phase 2C runbook + Live Demo README section

ci: add GitHub Actions workflow for python quality, tests, notebook freeze, and frontend build

ci: add workflow blocking AI attribution in commit messages