Add Cookbook other topics (draft)

docs-src/pages/en/cookbook/e03-sse-reconnect.md

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+---
+title: "E03. Handle SSE Reconnection"
+order: 49
+status: "draft"
+---
+
+SSE connections drop for all sorts of network reasons. Clients automatically try to reconnect, so it's a good idea to make your server resume from where it left off.
+
+## Read `Last-Event-ID`
+
+When the client reconnects, it sends the ID of the last event it received in the `Last-Event-ID` header. The server reads that and picks up from the next one.
+
+```cpp
+svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
+  auto last_id = req.get_header_value("Last-Event-ID");
+  int start = last_id.empty() ? 0 : std::stoi(last_id) + 1;
+
+  res.set_chunked_content_provider(
+    "text/event-stream",
+    [start](size_t offset, httplib::DataSink &sink) mutable {
+      static int next_id = 0;
+      if (next_id < start) { next_id = start; }
+
+      std::string msg = "id: " + std::to_string(next_id) + "\n"
+                      + "data: event " + std::to_string(next_id) + "\n\n";
+      sink.write(msg.data(), msg.size());
+      ++next_id;
+
+      std::this_thread::sleep_for(std::chrono::seconds(1));
+      return true;
+    });
+});
+```
+
+On the first connect, `Last-Event-ID` is empty, so start from `0`. On reconnect, resume from the next ID. Event history is the server's responsibility — you need to keep recent events around somewhere.
+
+## Set the reconnect interval
+
+Sending a `retry:` field tells the client how long to wait before reconnecting, in milliseconds.
+
+```cpp
+std::string msg = "retry: 5000\n\n";  // reconnect after 5 seconds
+sink.write(msg.data(), msg.size());
+```
+
+Usually you send this once at the start. During peak load or maintenance windows, a longer retry interval helps reduce reconnect storms.
+
+## Buffer recent events
+
+To support reconnection, keep a rolling buffer of recent events on the server.
+
+```cpp
+struct EventBuffer {
+  std::mutex mu;
+  std::deque<std::pair<int, std::string>> events; // {id, data}
+  int next_id = 0;
+
+  void push(const std::string &data) {
+    std::lock_guard<std::mutex> lock(mu);
+    events.push_back({next_id++, data});
+    if (events.size() > 1000) { events.pop_front(); }
+  }
+
+  std::vector<std::pair<int, std::string>> since(int id) {
+    std::lock_guard<std::mutex> lock(mu);
+    std::vector<std::pair<int, std::string>> out;
+    for (const auto &e : events) {
+      if (e.first >= id) { out.push_back(e); }
+    }
+    return out;
+  }
+};
+```
+
+When a client reconnects, call `since(last_id)` to send any events it missed.
+
+## How much to keep
+
+The buffer size is a tradeoff between memory and how far back a client can resume. It depends on the use case:
+
+- Real-time chat: a few minutes to half an hour
+- Notifications: the last N items
+- Trading data: persist to a database and pull from there
+
+> **Warning:** `Last-Event-ID` is a client-provided value — don't trust it blindly. If you read it as a number, validate the range. If it's a string, sanitize it.