fix(synthesizer): generate_one 捕获所有异常避免 VLM 超时穿透崩溃

except (ValueError, KeyError) → except Exception，
覆盖 StreamLivenessTimeout 等网络/超时异常。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/question_gen/synthesizer.py

@@ -737,7 +737,7 @@ async def generate_one(
                 source_nodes=(anchor.node_id,),
                 difficulty="medium",
             )
-        except (ValueError, KeyError) as exc:
+        except Exception as exc:
             logger.warning(
                 "generate_one 尝试 {}/{} 失败 ({}): {}",
                 attempt + 1,