Remove legacy fetch-data.js and gitignore generated static data (#72)

- Delete scripts/fetch-data.js (291 lines) — replaced by the pipeline
- Remove "fetch-data" npm script
- Remove static/data/*.json from git tracking — these are now build
  artifacts generated by the pipeline at deploy time
- Add /static/data/*.json to .gitignore

Note: fresh clones will need to run `npm run pipeline:build` to generate
local data files for development.

Closes #47.