feat: 添加租户工具配置系统

- 新增 platform_tool_configs 表和 ToolConfig Model
- 新增工具配置 CRUD API (/api/tool-configs)
- 租户详情页添加工具配置管理 Tab
- 修复查看 Token 显示问题，添加专用获取接口

backend/app/models/__init__.py

@@ -2,6 +2,7 @@
 from .tenant import Tenant, Subscription, Config
 from .tenant_app import TenantApp
 from .tenant_wechat_app import TenantWechatApp
+from .tool_config import ToolConfig
 from .app import App
 from .stats import AICallEvent, TenantUsageDaily
 from .logs import PlatformLog
@@ -14,6 +15,7 @@ __all__ = [
     "Config",
     "TenantApp",
     "TenantWechatApp",
+    "ToolConfig",
     "App",
     "AICallEvent",
     "TenantUsageDaily",