modify: auth插件拆分，将token签发部分交给网关

workers/auth_oauth/github_callback.toml

@@ -0,0 +1,12 @@
+[POST]
+summary = "GitHub 授权回调：换取用户信息并签发平台 token"
+docstring = "前端收到 GitHub 回跳的 code+state 后调用；校验 state（一次性）、换取用户信息、按绑定关系 find-or-create 账号，签发与内置登录同契约的平台 token。"
+auth_required = false
+cacheable = false
+params = ["code: str", "state: str"]
+
+[POST.response_model]
+result = { type = "str", required = true }
+token = { type = "str", required = true }
+message = { type = "str", required = true }
+timestamp = { type = "float", required = false }