docs: 存档 game_lucky_gift_count 时间窗口索引迁移

dashboard-cdc-worker 的 24h 窗口查询缺 create_time 可用索引，
每 5 分钟 examined ~18 万行；(sys_origin, create_time) 索引
已于 2026-07-10 在生产在线应用。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

migrations/057_game_lucky_gift_count_ctime_index.sql

@@ -0,0 +1,20 @@
+-- dashboard-cdc-worker 每 5 分钟按 create_time 24 小时窗口回读 game_lucky_gift_count（1400 万行），
+-- 原有索引均以 sys_origin,user_id 开头，无法按时间窗口定位，只能从 user_base_info 反向驱动全量探测
+-- （单次 examined ~18 万行，是线上 MySQL CPU 波动来源之一）。
+-- 本索引已于 2026-07-10 通过 ALGORITHM=INPLACE, LOCK=NONE 在生产手工应用（耗时 19s），此文件用于存档与新环境初始化。
+
+SET @current_schema := DATABASE();
+
+SET @add_game_lucky_gift_count_sorigin_ctime_sql := IF(
+  NOT EXISTS (
+    SELECT 1 FROM INFORMATION_SCHEMA.STATISTICS
+    WHERE table_schema = @current_schema
+      AND table_name = 'game_lucky_gift_count'
+      AND index_name = 'idx_game_lucky_gift_count_sorigin_ctime'
+  ),
+  'ALTER TABLE `game_lucky_gift_count` ADD INDEX `idx_game_lucky_gift_count_sorigin_ctime` (`sys_origin`, `create_time`), ALGORITHM=INPLACE, LOCK=NONE',
+  'SELECT 1'
+);
+PREPARE add_game_lucky_gift_count_sorigin_ctime_stmt FROM @add_game_lucky_gift_count_sorigin_ctime_sql;
+EXECUTE add_game_lucky_gift_count_sorigin_ctime_stmt;
+DEALLOCATE PREPARE add_game_lucky_gift_count_sorigin_ctime_stmt;