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/core/limiter.py

@@ -0,0 +1,4 @@
+from slowapi import Limiter
+from slowapi.util import get_remote_address
+
+limiter = Limiter(key_func=get_remote_address)