fix(governance): fail soft stale km readbacks

apps/api/src/api/v1/ai_governance.py

@@ -284,7 +284,25 @@ async def get_km_stale_candidates(
         project_id=project_id,
         limit=limit,
     )
-    return await query_km_stale_candidates(project_id=project_id, limit=limit)
+    try:
+        return await query_km_stale_candidates(project_id=project_id, limit=limit)
+    except Exception as exc:
+        logger.warning(
+            "km_stale_candidates_readback_degraded",
+            project_id=project_id,
+            limit=limit,
+            error_type=type(exc).__name__,
+        )
+        return KnowledgeStaleCandidatesResponse(
+            project_id=project_id,
+            total_stale=0,
+            returned=0,
+            threshold_days=0,
+            writes_on_read=False,
+            manual_review_required=True,
+            items=[],
+            generated_at=now_taipei(),
+        )
 
 
 # =============================================================================
@@ -405,6 +423,38 @@ async def get_km_stale_owner_review_completion_queue(
         )
     except KmStaleOwnerReviewError as exc:
         raise HTTPException(status_code=exc.status_code, detail=exc.detail) from exc
+    except Exception as exc:
+        logger.warning(
+            "km_stale_owner_review_completion_queue_readback_degraded",
+            project_id=project_id,
+            status_bucket=status_bucket,
+            priority_tiers=priority_tier,
+            recommended_completion_outcome=recommended_completion_outcome,
+            batch_governance_event_id=batch_governance_event_id,
+            can_preview=can_preview,
+            limit=limit,
+            error_type=type(exc).__name__,
+        )
+        return KnowledgeStaleOwnerReviewCompletionQueueResponse(
+            project_id=project_id,
+            status_bucket=status_bucket,
+            priority_tiers=priority_tier or [],
+            recommended_completion_outcome=recommended_completion_outcome,
+            batch_governance_event_id=batch_governance_event_id,
+            can_preview=can_preview,
+            total=0,
+            returned=0,
+            pending_count=0,
+            ready_count=0,
+            blocked_count=0,
+            completed_count=0,
+            failed_count=0,
+            writes_on_read=False,
+            manual_review_required=True,
+            batch_writes_allowed=False,
+            items=[],
+            generated_at=now_taipei(),
+        )
 
 
 # =============================================================================