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/run.py

@@ -0,0 +1,11 @@
+import uvicorn
+from app.core.config import get_settings
+
+if __name__ == "__main__":
+    settings = get_settings()
+    uvicorn.run(
+        "app.main:app",
+        host=settings.HOST,
+        port=settings.PORT,
+        reload=settings.DEBUG,
+    )