feat(backend): 意见反馈接口（提交+图片上传+admin列表）

- model/feedback.go: Feedback（多租户 shop_id，images 以 JSON 存）+ StringSlice 类型
- handler/feedback.go: Submit(认证，shop/user 从 JWT，去规范化 username/shop_code)、
  UploadImage(复用 product_image 压缩逻辑，存 /images/feedback/<shop>/)、
  List(SuperAdminOnly,分页过滤)、UpdateStatus(new/handled)
- router 注册 /feedback、/feedback/images、/admin/feedback；main AutoMigrate；schema.sql 加表
- feedback_test 覆盖提交/校验/权限/列表

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/schema/schema.sql

@@ -446,4 +446,26 @@ CREATE TABLE IF NOT EXISTS `product_spec_options` (
   KEY `idx_deleted_at` (`deleted_at`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='商品规格选项';
 
+CREATE TABLE IF NOT EXISTS `feedbacks` (
+  `id`          BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+  `created_at`  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  `shop_id`     BIGINT UNSIGNED NOT NULL,
+  `shop_code`   VARCHAR(50)  DEFAULT NULL,
+  `user_id`     BIGINT UNSIGNED DEFAULT NULL,
+  `username`    VARCHAR(50)  DEFAULT NULL,
+  `role`        VARCHAR(20)  DEFAULT NULL,
+  `type`        VARCHAR(20)  NOT NULL COMMENT 'bug / suggestion',
+  `content`     TEXT,
+  `images`      JSON,
+  `app_version` VARCHAR(30)  DEFAULT NULL,
+  `platform`    VARCHAR(20)  DEFAULT NULL,
+  `client_ip`   VARCHAR(60)  DEFAULT NULL,
+  `status`      VARCHAR(20)  NOT NULL DEFAULT 'new',
+  PRIMARY KEY (`id`),
+  KEY `idx_shop_id` (`shop_id`),
+  KEY `idx_user_id` (`user_id`),
+  KEY `idx_type` (`type`),
+  KEY `idx_status` (`status`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='用户意见反馈';
+
 SET FOREIGN_KEY_CHECKS = 1;