feat: 实现邮箱验证码和邮箱绑定功能

- 添加邮件发送服务实现(EmailService/EmailServiceImpl)
- 新增发送验证码、绑定邮箱、解绑邮箱接口
- 用户实体新增邮箱相关字段(userEmail/userEmailVerified)
- 添加邮件配置和JavaMailSender Bean
- 放行邮箱验证码接口(/v1/user/email/send-code)
- 新增ContextHolderUtils工具类用于获取当前用户上下文
- 完善Swagger文档注解

Co-Authored-By: Claude <noreply@anthropic.com>

aioj-backend-common/aioj-backend-common-core/pom.xml

@@ -67,5 +67,11 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-autoconfigure</artifactId>
         </dependency>
+        <!-- Spring Security (optional) -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-security</artifactId>
+            <optional>true</optional>
+        </dependency>
     </dependencies>
 </project>