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-auth/src/main/java/cn/meowrain/aioj/backend/auth/config/SecurityConfiguration.java

@@ -27,7 +27,7 @@ public class SecurityConfiguration {
 			.sessionManagement(session -> session.sessionCreationPolicy(SessionCreationPolicy.STATELESS))
 			.authorizeHttpRequests(auth -> auth
 				.requestMatchers("/v1/auth/**", "/oauth2/**", "/.well-known/**", "/doc.html", "/swagger-ui/**",
-						"/swagger-resources/**", "/webjars/**", "/v3/api-docs/**", "/v3/api-docs", "/favicon.ico")
+						"/swagger-resources/**", "/webjars/**", "/v3/api-docs/**", "/v3/api-docs", "/favicon.ico","/v1/user/email/send-code")
 				.permitAll()
 				.anyRequest()
 				.authenticated())