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/app/services/backtest/engine.py

@@ -0,0 +1,15 @@
+from typing import Optional, Dict, Any
+
+
+class BacktestEngine:
+    def __init__(self, config: Dict[str, Any]):
+        self.config = config
+
+    async def run(self) -> Dict[str, Any]:
+        raise NotImplementedError("Backtest engine not yet implemented")
+
+    async def stop(self):
+        raise NotImplementedError("Backtest stop not yet implemented")
+
+    def get_results(self) -> Dict[str, Any]:
+        raise NotImplementedError("Backtest results not yet implemented")