feat: 文件上传接口直接返回完整 URL + 提现 content 改为非必填

- FileUploadResponse / FileUploadCompleteResponse 精简为只返回 url
- S3Store.BuildObjectURL 拼接完整访问地址
- 提现 method=0 时 content 不再参与必填校验
- 新增用户端 API 接口文档

initialize/migrate/database/02152_withdrawal_method.up.sql

@@ -1,4 +1,10 @@
-ALTER TABLE `withdrawals`
-  ADD COLUMN `method`      TINYINT(1)   NOT NULL DEFAULT 0 COMMENT '收款方式 0:其他 1:支付宝 2:微信 3:银行卡' AFTER `content`,
-  ADD COLUMN `account`     VARCHAR(255) NOT NULL DEFAULT '' COMMENT '收款账号' AFTER `method`,
-  ADD COLUMN `qr_code_url` VARCHAR(500) NOT NULL DEFAULT '' COMMENT '收款码图片URL' AFTER `account`;
+SELECT COUNT(*) INTO @col_exists FROM INFORMATION_SCHEMA.COLUMNS
+WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = 'withdrawals' AND COLUMN_NAME = 'method';
+
+SET @ddl = IF(@col_exists = 0,
+  'ALTER TABLE `withdrawals` ADD COLUMN `method` TINYINT(1) NOT NULL DEFAULT 0 AFTER `content`, ADD COLUMN `account` VARCHAR(255) NOT NULL DEFAULT '''' AFTER `method`, ADD COLUMN `qr_code_url` VARCHAR(500) NOT NULL DEFAULT '''' AFTER `account`',
+  'SELECT 1');
+
+PREPARE stmt FROM @ddl;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;