Snapshot friend_back worktree for local merge

Source HEAD: 9d7e220f14
Includes uncommitted project changes and new project files.
Align shared text line endings and executable flags with Friendship-and-chat-api.
Normalize new text to LF; exclude .workbuddy local assistant memory.
Original friend_back checkout is unchanged.

sql/mysql/system/system_sub_account.sql

@@ -0,0 +1,17 @@
+-- 子账号管理：holder 与 子账号/专属角色 的绑定关系
+-- 执行位置：chat_v1 库
+CREATE TABLE IF NOT EXISTS `system_sub_account` (
+    `id`             bigint       NOT NULL AUTO_INCREMENT COMMENT '主键',
+    `holder_user_id` bigint       NOT NULL COMMENT '管理者用户编号（system_users.id）',
+    `sub_user_id`    bigint       NOT NULL COMMENT '子账号用户编号（system_users.id）',
+    `role_id`        bigint       NOT NULL COMMENT '子账号专属角色编号（system_role.id）',
+    `creator`        varchar(64)  NULL DEFAULT '' COMMENT '创建者',
+    `create_time`    datetime     NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
+    `updater`        varchar(64)  NULL DEFAULT '' COMMENT '更新者',
+    `update_time`    datetime     NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
+    `deleted`        bit(1)       NOT NULL DEFAULT b'0' COMMENT '是否删除',
+    `tenant_id`      bigint       NOT NULL DEFAULT 0 COMMENT '租户编号',
+    PRIMARY KEY (`id`),
+    KEY `idx_holder_user_id` (`holder_user_id`),
+    KEY `idx_sub_user_id` (`sub_user_id`)
+) ENGINE = InnoDB COMMENT '后台子账号表';