feat(backend): 实现 Gemini AI Studio OAuth 和消息兼容服务

- gemini_oauth_service.go: 新增 AI Studio OAuth 类型支持
- gemini_token_provider.go: Token 提供器增强
- gemini_messages_compat_service.go: 支持 AI Studio 端点
- account_test_service.go: Gemini 账户可用性检测
- gateway_service.go: 网关服务适配
- openai_gateway_service.go: OpenAI 兼容层调整

backend/internal/service/gateway_service.go

@@ -317,8 +317,17 @@ func (s *GatewayService) SelectAccountForModel(ctx context.Context, groupID *int
 			selected = acc
 		} else if acc.Priority == selected.Priority {
 			// 优先级相同时，选最久未用的
-			if acc.LastUsedAt == nil || (selected.LastUsedAt != nil && acc.LastUsedAt.Before(*selected.LastUsedAt)) {
+			switch {
+			case acc.LastUsedAt == nil && selected.LastUsedAt != nil:
 				selected = acc
+			case acc.LastUsedAt != nil && selected.LastUsedAt == nil:
+				// keep selected (never used is preferred)
+			case acc.LastUsedAt == nil && selected.LastUsedAt == nil:
+				// keep selected (both never used)
+			default:
+				if acc.LastUsedAt.Before(*selected.LastUsedAt) {
+					selected = acc
+				}
 			}
 		}
 	}