feat: add email_send_log table to track email sends and registration status

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/email_send_log.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS `email_send_log` (
+  `id` BIGINT NOT NULL AUTO_INCREMENT COMMENT '主键ID',
+  `email` VARCHAR(255) NOT NULL COMMENT '邮箱地址',
+  `status` TINYINT NOT NULL DEFAULT 0 COMMENT '状态: 0=已发送未注册, 1=已注册',
+  `created_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
+  `updated_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
+  PRIMARY KEY (`id`),
+  INDEX `idx_email` (`email`),
+  INDEX `idx_status` (`status`),
+  INDEX `idx_created_at` (`created_at`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='邮件发送记录表';