Add Docker Compose dev stack and local Supabase setup

From the PR description

Summary

• docker compose up now brings up frontend (:4000) and backend (:4001) with bind-mounted source for hot reload.
• supabase init scaffolding plus the existing one-shot schema copied into supabase/migrations/, so supabase start brings up local Postgres + Auth + Storage + Realtime and auto-applies the schema.
• README gains a "Run locally with Docker" section above the existing manual setup, including ports, env wiring (host.docker.internal for server-side, localhost for browser), and the one-time storage bucket bootstrap.

The container command runs npm install on start instead of relying on the image's baked node_modules. This works around a Docker Desktop quirk where a named volume mounted on top of a bind-mounted /app gets a partial copy of node_modules from the image (e.g. Next was missing format-cli-help-output).

Default ports moved to 4000/4001 to avoid colliding with anything else commonly running on 3000/3001. The legacy npm run dev workflow still uses 3000/3001 and is unaffected.

Test plan

• supabase start brings up the Supabase stack and applies the migration (15 tables in public)
• docker compose up -d starts both Mike services
• curl http://localhost:4001/health returns {"ok":true}
• curl http://localhost:4000/ redirects to /assistant (HTTP 200, ~30KB)
• Backend container reaches Supabase Auth via host.docker.internal:54321
• Source edits hot-reload inside both containers