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/ai_agent/crew.py

@@ -0,0 +1,15 @@
+from typing import List, Optional
+
+
+class CrewAgent:
+    def __init__(self, role: str, goal: str, backstory: str):
+        self.role = role
+        self.goal = goal
+        self.backstory = backstory
+
+    def execute_task(self, task: str) -> str:
+        raise NotImplementedError("CrewAI agent not yet implemented")
+
+
+def get_trading_crew():
+    raise NotImplementedError("Trading crew not yet implemented")