Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s02-json-api.md

@@ -0,0 +1,74 @@
+---
+title: "S02. Receive a JSON Request and Return a JSON Response"
+order: 21
+status: "draft"
+---
+
+cpp-httplib doesn't include a JSON parser. On the server side, combine it with something like [nlohmann/json](https://github.com/nlohmann/json). The examples below use `nlohmann/json`.
+
+## Receive and return JSON
+
+```cpp
+#include <httplib.h>
+#include <nlohmann/json.hpp>
+
+int main() {
+  httplib::Server svr;
+
+  svr.Post("/api/users", [](const httplib::Request &req, httplib::Response &res) {
+    try {
+      auto in = nlohmann::json::parse(req.body);
+
+      nlohmann::json out = {
+        {"id", 42},
+        {"name", in["name"]},
+        {"created_at", "2026-04-10T12:00:00Z"},
+      };
+
+      res.status = 201;
+      res.set_content(out.dump(), "application/json");
+    } catch (const std::exception &e) {
+      res.status = 400;
+      res.set_content("{\"error\":\"invalid json\"}", "application/json");
+    }
+  });
+
+  svr.listen("0.0.0.0", 8080);
+}
+```
+
+`req.body` is a plain `std::string`, so you pass it straight to your JSON library. For the response, `dump()` to a string and set the Content-Type to `application/json`.
+
+## Check the Content-Type
+
+```cpp
+svr.Post("/api/users", [](const httplib::Request &req, httplib::Response &res) {
+  auto content_type = req.get_header_value("Content-Type");
+  if (content_type.find("application/json") == std::string::npos) {
+    res.status = 415; // Unsupported Media Type
+    return;
+  }
+  // ...
+});
+```
+
+When you strictly want JSON only, verify the Content-Type up front.
+
+## A helper for JSON responses
+
+If you're writing the same pattern repeatedly, a small helper saves typing.
+
+```cpp
+auto send_json = [](httplib::Response &res, int status, const nlohmann::json &j) {
+  res.status = status;
+  res.set_content(j.dump(), "application/json");
+};
+
+svr.Get("/api/health", [&](const auto &req, auto &res) {
+  send_json(res, 200, {{"status", "ok"}});
+});
+```
+
+> **Note:** A large JSON body ends up entirely in `req.body`, which means it all sits in memory. For huge payloads, consider streaming reception — see S07. Receive Multipart Data as a Stream.
+
+> For the client side, see C02. Send and Receive JSON.