fix(tasks): align OutboxRelay with outbox_event status/next_attempt_at/idempotency_key

The relay only looked at published_at and ignored the ADR-002 §2.1 columns
added in db 0002_foundation: rows stayed status='pending' forever after
dispatch, failures were retried every tick without backoff, and the
row-level idempotency_key was never used as the engine enqueue key.

- claim status='pending' AND next_attempt_at<=now()
- enqueue with idempotency_key (fallback outbox:<id>)
- success: status=dispatched, published_at, attempts+1
- failure: attempts+1, exponential backoff (cap 5 min), last_error; dead at maxAttempts
- test helper table now mirrors db 0001+0002; tests cover backoff and dead

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/tasks/README.md

@@ -14,7 +14,7 @@
 | --- | --- |
 | `OpenWorkflowEngine.connect({ databaseUrl, db, registry })` | `TaskEngine` 实现:`enqueue / signal / cancel / get / list / startWorker / stop` |
 | `defineTask` / `TaskRegistry` / `createDefaultRegistry()` | 任务定义与注册表;默认注册表里有 §3.1-8 的 9 个任务(目前是空壳) |
-| `OutboxRelay` | 用 `FOR UPDATE SKIP LOCKED` 认领 `outbox_event` 里未投递的行,按 type 路由后 enqueue(幂等键 `outbox:<id>`),再标记 `published_at`;多进程并发时不会重复 |
+| `OutboxRelay` | 用 `FOR UPDATE SKIP LOCKED` 认领 `outbox_event` 中 `status=pending` 且 `next_attempt_at` 已到的行,按 type 路由后 enqueue(幂等键取行的 `idempotency_key`,缺省 `outbox:<id>`),成功标记 `dispatched` + `published_at`;失败 attempts+1 指数退避(上限 5 分钟),达 `maxAttempts` 置 `dead`;多进程并发时不会重复 |
 | `withAttemptToken(db, opKey, fn, { commit })` | 操作账本 fencing,表是 `tasks_op_ledger` |
 | `migrateTasksTables(db)` | 本包迁移(`migrations/`):`tasks_run_meta`、`tasks_fact`、`tasks_op_ledger`。W2 并入 `packages/db` |