feat: implement JWT authentication system

- Add register endpoint with bcrypt password hashing
- Add login endpoint returning JWT tokens
- Add logout endpoint with token blacklisting
- Add /me endpoint for current user info
- Add rate limiting (5/minute) for login attempts using slowapi
- Add user settings GET and PATCH endpoints
- Create auth middleware via get_current_user dependency
- Add UserSettings and UserSettingsUpdate schemas

src/backend/app/main.py

@@ -1,6 +1,9 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
+from slowapi import Limiter
+from slowapi.util import get_remote_address
 from .api import auth, bots, backtest, simulate, config
+from .core.limiter import limiter
 
 app = FastAPI(
     title="Randebu Trading Bot API",
@@ -8,6 +11,8 @@ app = FastAPI(
     version="0.1.0",
 )
 
+app.state.limiter = limiter
+
 app.add_middleware(
     CORSMiddleware,
     allow_origins=["*"],