[Day 6] FastAPI backend scaffold + Docker Compose

[nani3105]

Tasks\n- [ ] FastAPI project in apps/backend\n- [ ] PostgreSQL + SQLAlchemy models\n- [ ] Alembic migrations\n- [ ] docker-compose.yml\n- [ ] Health check\n\n## Acceptance Criteria\ndocker-compose up → API at localhost:8000/health\n\n> STRETCH GOAL — skip if Day 1–5 not complete