step 1 complete: runnable FastAPI skeleton

- Full directory structure with all __init__.py stubs
- requirements.txt with all pinned dependencies
- app/config/settings.py (BaseSettings, env-based)
- app/main.py (CORS, lifespan, /api/v1/health)
- Dockerfile (multi-stage, Python 3.12-slim, non-root user)
- docker-compose.yml (app + postgres:16 with healthcheck)
- .env.example
- BACKEND_PLAN.md: mark step 1 done, add one-step-at-a-time rule

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,19 @@
+fastapi>=0.115.0
+uvicorn[standard]>=0.34.0
+langchain>=0.3.0
+langchain-openai>=0.3.0
+pydantic>=2.10.0
+pydantic-settings>=2.7.0
+python-jose[cryptography]>=3.3.0
+stripe>=11.0.0
+boto3>=1.35.0
+slowapi>=0.1.9
+sqlalchemy>=2.0.0
+asyncpg>=0.30.0
+alembic>=1.14.0
+bcrypt>=4.2.0
+python-dotenv>=1.0.0
+httpx>=0.28.0
+websockets>=14.0
+pytest>=8.0.0
+pytest-asyncio>=0.24.0