feat: backend project setup with FastAPI structure and dependencies

- Create directory structure per IMPLEMENTATION_PLAN.md Section 12
- Add requirements.txt with FastAPI, SQLAlchemy, CrewAI, etc.
- Add core/config.py for environment variable configuration
- Add core/database.py for SQLite connection
- Add core/security.py for password hashing and JWT
- Add FastAPI app entry point (main.py) with all API routers
- Add Uvicorn runner (run.py)
- Add API route stubs (auth, bots, backtest, simulate, config)
- Add db/models.py with SQLAlchemy models
- Add db/schemas.py with Pydantic schemas
- Add service stubs (ai_agent, backtest, simulate engines)
- Add .env.example with all required environment variables
- Verify server starts correctly

src/backend/.env.example

@@ -0,0 +1,11 @@
+DATABASE_URL=sqlite:///./data/app.db
+SECRET_KEY=your-super-secret-key-change-in-production
+JWT_ALGORITHM=HS256
+ACCESS_TOKEN_EXPIRE_MINUTES=1440
+MINIMAX_API_KEY=your-minimax-api-key
+MINIMAX_MODEL=MiniMax-Text-01
+AVE_API_KEY=your-ave-cloud-api-key
+AVE_API_PLAN=free
+HOST=0.0.0.0
+PORT=8000
+DEBUG=false