fix(user-service): resolve Spring Cloud OpenFeign and Jakarta servlet dependencies

- Add spring-cloud-starter-openfeign dependency for Feign client support
- Add jakarta.servlet-api dependency for Spring Boot 3.x compatibility
- Update HttpServletRequest import from javax.servlet to jakarta.servlet in SmartCollectionController
- Document backend build fixes and dependency migration in BACKEND_FIXES_SUMMARY.md
- Resolve compilation errors caused by missing Spring Cloud and Jakarta EE dependencies

timeline-user-service/src/main/java/com/timeline/user/controller/SmartCollectionController.java

@@ -9,7 +9,7 @@ import lombok.extern.slf4j.Slf4j;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.web.bind.annotation.*;
 
-import javax.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletRequest;
 import java.util.List;
 
 /**