Adds an opt-in compose override that bind-mounts the three_cx provider
files (and the alembic versions dir, so newer local migrations resolve)
into the prebuilt API image — useful when iterating on a provider
without rebuilding. Loaded explicitly via
`docker compose -f docker-compose.yaml -f docker-compose.dev.yaml up`.
Also blanks the default `NEXT_PUBLIC_BACKEND_URL` in `ui/.env.example`
so the browser uses `window.location.origin` and the Next.js rewrite in
`next.config.ts` proxies `/api/v1/*` to the backend. This works for both
single-host dev and remote dev (where `localhost:8000` from the browser
points at the wrong machine).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
