Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s12-user-data.md

@@ -0,0 +1,56 @@
+---
+title: "S12. Pass Data Between Handlers with res.user_data"
+order: 31
+status: "draft"
+---
+
+Say your pre-request handler decodes an auth token and you want the route handler to use the result. That "data handoff between handlers" is what `res.user_data` is for — it holds values of arbitrary types.
+
+## Basic usage
+
+```cpp
+struct AuthUser {
+  std::string id;
+  std::string name;
+  bool is_admin;
+};
+
+svr.set_pre_request_handler(
+  [](const httplib::Request &req, httplib::Response &res) {
+    auto token = req.get_header_value("Authorization");
+    auto user = decode_token(token); // decode the auth token
+    res.user_data.set("user", user);
+    return httplib::Server::HandlerResponse::Unhandled;
+  });
+
+svr.Get("/me", [](const httplib::Request &req, httplib::Response &res) {
+  auto *user = res.user_data.get<AuthUser>("user");
+  if (!user) {
+    res.status = 401;
+    return;
+  }
+  res.set_content("Hello, " + user->name, "text/plain");
+});
+```
+
+`user_data.set()` stores a value of any type, and `user_data.get<T>()` retrieves it. If you give the wrong type you get `nullptr` back — so be careful.
+
+## Typical value types
+
+Strings, numbers, structs, `std::shared_ptr` — anything copyable or movable works.
+
+```cpp
+res.user_data.set("user_id", std::string{"42"});
+res.user_data.set("is_admin", true);
+res.user_data.set("started_at", std::chrono::steady_clock::now());
+```
+
+## Where to set, where to read
+
+The usual flow is: set it in `set_pre_routing_handler()` or `set_pre_request_handler()`, read it in the route handler. Pre-request runs after routing, so you can combine it with `req.matched_route` to set values only for specific routes.
+
+## A gotcha
+
+`user_data` lives on `Response`, not `Request`. That's because handlers get `Response&` (mutable) but only `const Request&`. It looks odd at first, but it makes sense once you think of it as "the mutable context shared between handlers."
+
+> **Warning:** `user_data.get<T>()` returns `nullptr` when the type doesn't match. Use the exact same type on set and get. Storing as `AuthUser` and fetching as `const AuthUser` won't work.