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/requirements.txt

@@ -10,3 +10,4 @@ crewai>=0.1.0
 anthropic>=0.18.0
 httpx>=0.26.0
 python-multipart>=0.0.6
+slowapi>=0.1.9