fix(km): expose degraded readback reason

apps/api/tests/test_knowledge_service_readback_degraded.py

@@ -60,6 +60,8 @@ async def test_knowledge_list_entries_fails_soft_when_readback_breaks(monkeypatc
     ]
     assert all(row.count >= 1 for row in response.asset_taxonomy)
     assert response.readback_status == "source_backed_degraded"
+    assert response.primary_readback_ready is False
+    assert response.degraded_reason_code == "primary_km_db_timeout_or_pool_exhausted"
     assert response.operator_stage == "knowledge_readback_source_backed_ai_controlled_repair"
     assert response.next_step == "repair_primary_km_db_readback_then_promote_source_backed_receipts_to_persistent_km"
     assert response.writes_on_read is False
@@ -83,6 +85,26 @@ async def test_knowledge_list_entries_source_backed_filter_and_search(monkeypatc
     assert response.asset_taxonomy[[row.key for row in response.asset_taxonomy].index("alert")].count == 1
 
 
+@pytest.mark.asyncio
+async def test_knowledge_search_fails_soft_to_source_backed_entries(monkeypatch) -> None:
+    monkeypatch.setattr(
+        knowledge_service_module,
+        "get_db_context",
+        lambda: _BrokenDbContext(),
+    )
+    service = KnowledgeService.__new__(KnowledgeService)
+
+    entries = await service.search("Telegram", limit=10)
+
+    entry_ids = {entry.id for entry in entries}
+    assert all(entry.id.startswith("source-backed-") for entry in entries)
+    assert {
+        "source-backed-service-telegram-alert-receipts",
+        "source-backed-alert-telegram-monitoring-coverage",
+        "source-backed-schedule-report-monitoring",
+    }.issubset(entry_ids)
+
+
 @pytest.mark.asyncio
 async def test_knowledge_categories_fails_soft_when_readback_breaks(monkeypatch) -> None:
     monkeypatch.setattr(