Add Docker Compose dev stack and local Supabase setup

Two services in docker-compose.yml (frontend on :4000, backend on :4001)
with bind-mounted source for hot reload. Container command runs `npm install`
on start to work around a Docker Desktop named-volume init quirk where
node_modules from the image gets a partial copy when overlapped by a bind
mount.

`supabase init` scaffolding with the existing one-shot schema copied into
supabase/migrations/ so `supabase start` brings up Postgres + Auth +
Storage + Realtime locally and applies the schema automatically.

README: new "Run locally with Docker" section covering ports, env wiring
(host.docker.internal for server-side, localhost for browser), and the
storage bucket bootstrap.