feat: sample terminal business latency

internal/storage/mysql/migrations/000028_terminal_latency_metrics.up.sql

@@ -0,0 +1,7 @@
+-- 为终态任务/流程的最近完成耗时样本提供固定索引。
+-- 采样只读已持久化时间，不修改状态、不引入外键或级联。
+ALTER TABLE pulse_tasks
+    ADD KEY ix_task_observe_terminal_completed (state, completed_at, id);
+
+ALTER TABLE weave_instances
+    ADD KEY ix_workflow_observe_terminal_completed (state, completed_at, id);