docs: add project README

README.md

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+# Distributed Workflow Engine
+
+A distributed workflow engine built with FastAPI, SQLAlchemy 2.0 (async), and Python 3.12+.
+
+## Features
+
+- **Async API** — FastAPI with app factory pattern and lifespan context manager
+- **Authentication** — JWT access/refresh tokens with Argon2 password hashing
+- **Workflow CRUD** — Create, read, update, and delete workflows scoped per user
+- **Async Database** — SQLAlchemy 2.0 async with PostgreSQL (asyncpg) in production, SQLite (aiosqlite) in tests
+- **Observability** — Structured logging with structlog
+- **Migrations** — Alembic for database schema management
+- **CI/CD** — GitHub Actions with ruff linting, mypy type checking, and pytest
+
+## Project Structure
+
+```
+backend/
+├── dwf/
+│   ├── main.py                  # App factory + lifespan
+│   ├── settings.py              # Pydantic Settings (env-based config)
+│   ├── api/
+│   │   ├── deps.py              # Shared dependencies (get_db, get_current_user)
+│   │   └── routers/
+│   │       ├── auth.py          # Register, login, refresh
+│   │       ├── health.py        # Health check endpoint
+│   │       └── workflows.py     # Workflow CRUD endpoints
+│   ├── domain/models/
+│   │   ├── user.py              # Pydantic schemas for auth
+│   │   └── workflow.py          # Pydantic schemas + WorkflowStatus enum
+│   ├── infrastructure/database/
+│   │   ├── engine.py            # Async engine + session factory
+│   │   └── models/
+│   │       ├── base.py          # SQLAlchemy DeclarativeBase
+│   │       ├── user.py          # User table
+│   │       └── workflow.py      # Workflow table
+│   └── services/
+│       ├── auth_service.py      # Auth logic (register, login, JWT)
+│       └── workflow_service.py  # Workflow CRUD logic
+├── tests/
+│   ├── conftest.py              # Async fixtures (engine, session, client)
+│   └── unit/
+│       ├── test_auth.py         # 8 auth tests
+│       └── test_workflow.py     # 8 workflow tests
+├── migration/                   # Alembic migrations
+├── Dockerfile
+├── requirements.txt
+└── pyproject.toml
+```
+
+## Getting Started
+
+### Prerequisites
+
+- Python 3.12+
+- PostgreSQL (or Neon for serverless)
+
+### Setup
+
+```bash
+cd backend
+
+# Create virtual environment
+python -m venv .venv
+source .venv/bin/activate
+
+# Install dependencies
+pip install -r requirements.txt
+
+# Configure environment
+cp .env.example .env
+# Edit .env with your DATABASE_URL and JWT_SECRET_KEY
+```
+
+### Environment Variables
+
+| Variable | Description | Example |
+|---|---|---|
+| `DATABASE_URL` | Async database connection string | `postgresql+asyncpg://user:pass@host/db` |
+| `JWT_SECRET_KEY` | Secret key for JWT signing | `openssl rand -hex 32` |
+| `LOG_LEVEL` | Logging level (default: `info`) | `debug` |
+
+### Run
+
+```bash
+uvicorn dwf.main:app --reload
+```
+
+API docs available at `http://localhost:8000/docs`.
+
+### Test
+
+```bash
+cd backend
+pytest tests/ -v
+```
+
+### Lint
+
+```bash
+ruff check backend/
+ruff format --check backend/
+```
+
+### Docker
+
+```bash
+docker compose build
+docker compose up
+```
+
+## API Endpoints
+
+| Method | Path | Description | Auth |
+|---|---|---|---|
+| `GET` | `/health` | Health check | No |
+| `POST` | `/auth/register` | Register new user | No |
+| `POST` | `/auth/login` | Login, get tokens | No |
+| `POST` | `/auth/refresh` | Refresh access token | No |
+| `POST` | `/workflows/` | Create workflow | Yes |
+| `GET` | `/workflows/` | List user's workflows | Yes |
+| `GET` | `/workflows/{id}` | Get workflow by ID | Yes |
+| `PUT` | `/workflows/{id}` | Update workflow | Yes |
+| `DELETE` | `/workflows/{id}` | Delete workflow | Yes |
+
+## Tech Stack
+
+- **Framework**: FastAPI
+- **ORM**: SQLAlchemy 2.0 (async)
+- **Database**: PostgreSQL + asyncpg
+- **Auth**: PyJWT + Argon2
+- **Validation**: Pydantic v2
+- **Config**: pydantic-settings
+- **Logging**: structlog
+- **Migrations**: Alembic
+- **Testing**: pytest + pytest-asyncio + httpx + aiosqlite
+- **Linting**: ruff + mypy
+- **CI**: GitHub Actions