fix(km): retry transient list readbacks

apps/api/src/services/knowledge_service.py

@@ -462,6 +462,57 @@ class KnowledgeService:
             repo: IKnowledgeRepository = KnowledgeDBRepository(db)
             return await repo.delete(entry_id)
 
+    async def _list_entries_from_primary(
+        self,
+        *,
+        category: str | None,
+        entry_type: EntryType | None,
+        status: EntryStatus | None,
+        tags: list[str] | None,
+        q: str | None,
+        limit: int,
+        offset: int,
+    ) -> KnowledgeListResponse:
+        async with get_db_context() as db:
+            repo: IKnowledgeRepository = KnowledgeDBRepository(db)
+            items, total = await repo.list_entries(
+                category=category,
+                entry_type=entry_type,
+                status=status,
+                tags=tags,
+                q=q,
+                limit=limit,
+                offset=offset,
+            )
+            categories_raw = await repo.get_categories()
+            categories = [
+                CategoryCount(category=cat, count=cnt) for cat, cnt in categories_raw
+            ]
+            asset_taxonomy_raw = await repo.get_asset_taxonomy_counts()
+            asset_taxonomy = [
+                KnowledgeAssetTaxonomyCount(key=key, count=count)
+                for key, count in asset_taxonomy_raw
+            ]
+            if total == 0:
+                return build_knowledge_list_readback_degraded_response(
+                    "knowledge_db_empty",
+                    category=category,
+                    entry_type=entry_type,
+                    status=status,
+                    tags=tags,
+                    q=q,
+                    limit=limit,
+                    offset=offset,
+                    readback_status="source_backed_db_empty",
+                )
+            return KnowledgeListResponse(
+                items=items,
+                total=total,
+                categories=categories,
+                asset_taxonomy=asset_taxonomy,
+                primary_readback_ready=True,
+            )
+
     async def list_entries(
         self,
         category: str | None = None,
@@ -474,29 +525,21 @@ class KnowledgeService:
     ) -> KnowledgeListResponse:
         """列出知識條目 + 分類統計"""
         try:
-            async with get_db_context() as db:
-                repo: IKnowledgeRepository = KnowledgeDBRepository(db)
-                items, total = await repo.list_entries(
-                    category=category,
-                    entry_type=entry_type,
-                    status=status,
-                    tags=tags,
-                    q=q,
-                    limit=limit,
-                    offset=offset,
-                )
-                categories_raw = await repo.get_categories()
-                categories = [
-                    CategoryCount(category=cat, count=cnt) for cat, cnt in categories_raw
-                ]
-                asset_taxonomy_raw = await repo.get_asset_taxonomy_counts()
-                asset_taxonomy = [
-                    KnowledgeAssetTaxonomyCount(key=key, count=count)
-                    for key, count in asset_taxonomy_raw
-                ]
-                if total == 0:
-                    return build_knowledge_list_readback_degraded_response(
-                        "knowledge_db_empty",
+            return await self._list_entries_from_primary(
+                category=category,
+                entry_type=entry_type,
+                status=status,
+                tags=tags,
+                q=q,
+                limit=limit,
+                offset=offset,
+            )
+        except Exception as exc:  # noqa: BLE001 - production readback must fail soft
+            degraded_reason_code = _classify_knowledge_readback_degraded_reason(str(exc))
+            if degraded_reason_code == "primary_km_db_timeout_or_pool_exhausted":
+                try:
+                    await asyncio.sleep(0.25)
+                    retry_response = await self._list_entries_from_primary(
                         category=category,
                         entry_type=entry_type,
                         status=status,
@@ -504,19 +547,22 @@ class KnowledgeService:
                         q=q,
                         limit=limit,
                         offset=offset,
-                        readback_status="source_backed_db_empty",
                     )
-                return KnowledgeListResponse(
-                    items=items,
-                    total=total,
-                    categories=categories,
-                    asset_taxonomy=asset_taxonomy,
-                    primary_readback_ready=True,
-                )
-        except Exception as exc:  # noqa: BLE001 - production readback must fail soft
+                    retry_response.operator_stage = "knowledge_readback_primary_retry_recovered"
+                    return retry_response
+                except Exception as retry_exc:  # noqa: BLE001 - keep source-backed fallback
+                    logger.warning(
+                        "knowledge_list_readback_retry_degraded",
+                        error=str(retry_exc),
+                        degraded_reason_code=_classify_knowledge_readback_degraded_reason(str(retry_exc)),
+                        q=q,
+                        limit=limit,
+                        offset=offset,
+                    )
             logger.warning(
                 "knowledge_list_readback_degraded",
                 error=str(exc),
+                degraded_reason_code=degraded_reason_code,
                 category=category,
                 entry_type=entry_type.value if entry_type else None,
                 status=status.value if status else None,