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/pom.xml

@@ -43,6 +43,12 @@
             <groupId>org.springframework.cloud</groupId>
             <artifactId>spring-cloud-starter-bootstrap</artifactId>
         </dependency>
+        
+        <!-- Spring Cloud OpenFeign -->
+        <dependency>
+            <groupId>org.springframework.cloud</groupId>
+            <artifactId>spring-cloud-starter-openfeign</artifactId>
+        </dependency>
 
         <!-- Spring Cloud Alibaba Nacos -->
         <dependency>
@@ -53,6 +59,13 @@
             <groupId>com.alibaba.cloud</groupId>
             <artifactId>spring-cloud-starter-alibaba-nacos-config</artifactId>
         </dependency>
+        
+        <!-- Jakarta Servlet API (replaces javax.servlet in Spring Boot 3) -->
+        <dependency>
+            <groupId>jakarta.servlet</groupId>
+            <artifactId>jakarta.servlet-api</artifactId>
+            <scope>provided</scope>
+        </dependency>
 
         <!-- JWT -->
         <dependency>