feat(usage): 添加清理任务与统计过滤

backend/migrations/043_add_usage_cleanup_cancel_audit.sql

@@ -0,0 +1,10 @@
+-- 043_add_usage_cleanup_cancel_audit.sql
+-- usage_cleanup_tasks 取消任务审计字段
+
+ALTER TABLE usage_cleanup_tasks
+    ADD COLUMN IF NOT EXISTS canceled_by BIGINT REFERENCES users(id) ON DELETE SET NULL,
+    ADD COLUMN IF NOT EXISTS canceled_at TIMESTAMPTZ;
+
+CREATE INDEX IF NOT EXISTS idx_usage_cleanup_tasks_canceled_at
+    ON usage_cleanup_tasks(canceled_at DESC);
+