feat: sync group call livestream support

deploy/migrations/0054_group_call_scheduled.up.sql

@@ -0,0 +1,10 @@
+-- Scheduled video chat：group_calls 增加 schedule_date（>0=定时未开始）；
+-- 开播提醒订阅（toggleGroupCallStartSubscription 的 per-user 状态）。
+ALTER TABLE group_calls
+    ADD COLUMN IF NOT EXISTS schedule_date integer DEFAULT 0 NOT NULL;
+
+CREATE TABLE IF NOT EXISTS group_call_schedule_subscribers (
+    call_id bigint NOT NULL,
+    user_id bigint NOT NULL,
+    PRIMARY KEY (call_id, user_id)
+);