Add Makefile for common developer tasks

[artugro]

Context

No automation for common developer actions. New contributors have to read through README and CLAUDE.md to piece together the setup steps. A Makefile provides discoverable, standardized entry points.

Proposed Targets

make setup      # Create venv, install deps, start docker services, run migrations
make test       # Run pytest with standard flags
make lint       # Run ruff check + ruff format --check
make format     # Run ruff format (auto-fix)
make dev        # Start uvicorn with --reload on port 8000
make docker     # Build and run full stack via docker-compose
make migrate    # Run alembic upgrade head
make openapi    # Export OpenAPI spec to docs/openapi.json
make clean      # Remove __pycache__, .pytest_cache, etc.

Why

• Reduces onboarding friction from ~10 minutes to make setup && make dev
• Standardizes commands across contributors
• Self-documenting — make help lists all targets