Add docker-compose.yml for one-command local setup

[KonstantinMB]

Right now setting up local dev requires Python 3.11+, pip, Node 20+, npm, and optionally a Postgres instance. A docker-compose.yml that brings up Postgres + backend + frontend with one command would cut first-time setup from ~15 minutes to ~60 seconds.

What to do

• Add docker-compose.yml at the repo root with three services:
  • postgres — Postgres 16 with a named volume for persistence, exposes :5432
  • backend — builds from backend/Dockerfile (new file), exposes :8000, depends on postgres, env vars wired up
  • frontend — builds from frontend/Dockerfile (new file), exposes :5173
• Add backend/Dockerfile (Python 3.11 slim, install requirements, run uvicorn)
• Add frontend/Dockerfile (Node 20 alpine, install deps, run npm run dev with host binding for Docker)
• DATABASE_URL in compose should point at the Postgres service hostname (e.g. postgresql://postgres:postgres@postgres:5432/exploreyc)
• Update README "Quick start" with a Docker alternative below the existing instructions (don't replace them — keep both)

Acceptance criteria

• docker compose up (no separate build step) starts everything
• Frontend reachable at http://localhost:5173
• Backend reachable at http://localhost:8000 with the API responding
• Database persists across docker compose down && docker compose up
• .env.example updated if any new env vars are introduced
• Works on macOS, Linux, and Windows (WSL2)

Pointers

• backend/main.py reads DATABASE_URL via backend/database_factory.py — point at the Postgres service name
• Migrations live in supabase/migrations/ — decide whether compose runs them automatically or whether contributors run them manually; document either way
• Vite dev server needs --host 0.0.0.0 to be reachable from outside the container — see frontend/vite.config.ts

Scope

Touches 3-4 files (compose, 2 Dockerfiles, README). Estimated time: 2-4 hours.

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