fix(leonardo): cookie 写回改 CAS，并补上额度对账后的轮换写回

Leonardo 只认加密的 session_data 缓存，轮换值一旦没存住账号就会被判死。写回原先是无条件 update：长任务（生成、慢额度查询）手里的旧 cookie 完成时会盖掉期间 keepalive 存好的新值。改为 SwapValue（WHERE value = 旧值 才写），旧值无法覆盖新值；reconcileLeonardoCredits 原先完全没写回 FetchCreditsBalance 带回的轮换值，补上。

backend/internal/service/tokens.go

@@ -195,10 +195,10 @@ func (s *TokenService) RefreshLeonardoSessions(ctx context.Context) {
 				}
 				continue // leave the stamp alone so the next tick retries
 			}
-			fields := map[string]any{}
 			if fresh, ok := s.leonardo.RotatedCookie(a.Value); ok && strings.TrimSpace(fresh) != "" {
-				fields["value"] = fresh
+				_, _ = s.tokens.SwapValue(callCtx, "leonardo", a.ID, a.Value, fresh)
 			}
+			fields := map[string]any{}
 			meta := cloneJSONMap(a.Meta)
 			meta[leonardoKeptAtKey] = int(time.Now().Unix())
 			fields["meta"] = meta
@@ -459,7 +459,7 @@ func (s *TokenService) persistLeonardoCookie(ctx context.Context, tokenID, cooki
 		return
 	}
 	if fresh, ok := s.leonardo.RotatedCookie(cookie); ok && strings.TrimSpace(fresh) != "" {
-		_, _ = s.tokens.Update(ctx, "leonardo", tokenID, map[string]any{"value": fresh})
+		_, _ = s.tokens.SwapValue(ctx, "leonardo", tokenID, cookie, fresh)
 	}
 }