fix: add node_bundler=esbuild for jas-tags and stretched-paintings functions
The nanoid v4+ package is ESM-only and breaks when Netlify's default
bundler wraps it in require(). Adding esbuild bundler config (like other
functions) fixes the 502 errors.
Also reverts API_BASE to relative paths since domain-scoped redirects
in netlify.toml handle routing correctly.