fix(km): keep backfill reconciler loop alive

apps/api/src/jobs/km_backfill_reconciler_job.py

@@ -25,7 +25,9 @@ Feature Flag：
 
 from __future__ import annotations
 
+import asyncio
 import json
+
 import structlog
 
 from src.core.config import settings

apps/api/tests/test_km_writer_backfill_reconciler.py

@@ -16,19 +16,20 @@ P1-1 C1 修復 2026-04-28 ogt + Claude Sonnet 4.6
 """
 
 import json
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
+import src.jobs.km_backfill_reconciler_job as reconciler_job
 from src.jobs.km_backfill_reconciler_job import (
     run_km_backfill_reconciler,
+    run_km_backfill_reconciler_loop,
 )
 from src.services.km_writer import (
     KM_BACKFILL_DLQ_KEY,
     _backfill_path_a_approval_safe,
 )
 
-
 # =============================================================================
 # Helper
 # =============================================================================
@@ -136,6 +137,32 @@ async def test_reconciler_empty_dlq():
     assert result["failed"] == 0
 
 
+@pytest.mark.asyncio
+async def test_reconciler_loop_can_sleep(monkeypatch: pytest.MonkeyPatch):
+    """loop 必須能 sleep；避免少 import asyncio 導致 background task 啟動即死亡。"""
+
+    class StopLoop(Exception):
+        pass
+
+    calls = 0
+
+    async def fake_run_once():
+        nonlocal calls
+        calls += 1
+        return {"processed": 0, "success": 0, "failed": 0}
+
+    async def fake_sleep(_seconds: int):
+        raise StopLoop
+
+    monkeypatch.setattr(reconciler_job, "run_km_backfill_reconciler", fake_run_once)
+    monkeypatch.setattr(reconciler_job.asyncio, "sleep", fake_sleep)
+
+    with pytest.raises(StopLoop):
+        await run_km_backfill_reconciler_loop()
+
+    assert calls == 1
+
+
 # =============================================================================
 # 5. ENABLE_KM_BACKFILL_RECONCILER=false → 跳過
 # =============================================================================

docs/LOGBOOK.md

@@ -3858,3 +3858,33 @@ ruff check apps/api/src/core/logging.py apps/api/src/services/failover_alerter.p
 ### 注意
 
 - `telegram_gateway.py` 全檔仍有大量既有 ruff 債，本次只針對 token 外洩與 MarkdownV2 400 風險做最小安全修補，避免在 6000+ 行 gateway 巨檔混入無關機械改動。
+
+---
+
+## 2026-05-06（台北）— KM backfill reconciler background loop 修復
+
+**觸發**：production API 啟動後出現 `Task exception was never retrieved`，`run_km_backfill_reconciler_loop()` 因 `NameError: name 'asyncio' is not defined` 在第一次 sleep 前死亡，導致 `km:backfill:dlq` 補救 loop 沒有持續運作。
+
+### 已修正
+
+| 範圍 | 結果 |
+|------|------|
+| `km_backfill_reconciler_job.py` | 補上 `import asyncio`，讓 5 分鐘循環 sleep 可正常執行 |
+| 回歸測試 | 新增 `test_reconciler_loop_can_sleep()`，用 fake sleep 主動中止 loop，驗證 loop 至少能跑一次 reconciler 並進入 sleep |
+
+### 驗證
+
+```text
+pytest apps/api/tests/test_km_writer_backfill_reconciler.py -q
+# 8 passed
+
+py_compile apps/api/src/jobs/km_backfill_reconciler_job.py apps/api/tests/test_km_writer_backfill_reconciler.py
+# 通過
+
+ruff check apps/api/src/jobs/km_backfill_reconciler_job.py apps/api/tests/test_km_writer_backfill_reconciler.py
+# All checks passed
+```
+
+### 影響
+
+- KM / PlayBook / RAG 飛輪的 backfill 補救鏈恢復可持續執行，避免 DLQ 堆積後造成知識庫關聯缺口。