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/core/config.py

@@ -0,0 +1,25 @@
+from pydantic_settings import BaseSettings
+from functools import lru_cache
+
+
+class Settings(BaseSettings):
+    DATABASE_URL: str = "sqlite:///./data/app.db"
+    SECRET_KEY: str
+    JWT_ALGORITHM: str = "HS256"
+    ACCESS_TOKEN_EXPIRE_MINUTES: int = 1440
+    MINIMAX_API_KEY: str
+    MINIMAX_MODEL: str = "MiniMax-Text-01"
+    AVE_API_KEY: str
+    AVE_API_PLAN: str = "free"
+    HOST: str = "0.0.0.0"
+    PORT: int = 8000
+    DEBUG: bool = False
+
+    class Config:
+        env_file = ".env"
+        extra = "allow"
+
+
+@lru_cache()
+def get_settings() -> Settings:
+    return Settings()