feat: support durable task cancellation without foreign keys

internal/storage/mysql/migrations/000007_task_cancellation.up.sql

@@ -0,0 +1,4 @@
+ALTER TABLE pulse_attempt_evidence
+ DROP CHECK ck_attempt_evidence_outcome,
+ MODIFY COLUMN outcome VARCHAR(16) NOT NULL COMMENT 'Worker 声明的结果：SUCCEEDED、FAILED、UNKNOWN 或已确认停止且本次无遗留效果的 CANCELLED',
+ ADD CONSTRAINT ck_attempt_evidence_outcome CHECK(outcome IN ('SUCCEEDED','FAILED','UNKNOWN','CANCELLED'));