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/api/config.py

@@ -0,0 +1,13 @@
+from fastapi import APIRouter
+
+router = APIRouter()
+
+
+@router.get("/chains")
+def get_chains():
+    return {"chains": []}
+
+
+@router.get("/tokens")
+def get_tokens():
+    return {"tokens": []}