fix: 群写操作在事务内复核并去重建群成员

internal/store/migrations/0003_group_members_fk.sql

@@ -0,0 +1,19 @@
+-- U-02: group_members 增加指向 groups 的外键，避免解散后残留孤儿行。
+-- 本分支基于 C-04 时最大迁移号为 0002，按 TASKS 4.2 取 0003。
+DELETE FROM group_members WHERE group_id NOT IN (SELECT id FROM groups);
+
+CREATE TABLE group_members_new (
+  group_id TEXT NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
+  endpoint_id TEXT NOT NULL,
+  joined_at INTEGER NOT NULL,
+  PRIMARY KEY (group_id, endpoint_id)
+);
+
+INSERT INTO group_members_new (group_id, endpoint_id, joined_at)
+SELECT group_id, endpoint_id, joined_at FROM group_members;
+
+DROP TABLE group_members;
+
+ALTER TABLE group_members_new RENAME TO group_members;
+
+CREATE INDEX idx_group_members_endpoint ON group_members(endpoint_id);