Fix README and docs pages that no longer match the code

Bring the READMEs and the docs pages (en and ja) in line with the
current header:

- Fix the client sample that does not compile, the Transfer-Encoding
  statement for default headers, the verifier callback comment, the
  thread pool spawn rule, the sanitize_filename description, and the
  inverted Accept-Encoding sentence.
- Describe recent behaviour: TLS errors behind Read/Write failures,
  Host validation, Connection: close for a body without length,
  content_reader rules, refused CONNECT, WebSocketClient proxies,
  CPPHTTPLIB_INTERIM_RESPONSE_MAX_COUNT, the SSE event size limit, and
  the payload limit of the Stream API.
- Complete the API references of WebSocket, WebSocketClient and the
  stream:: helpers, and the SSE reconnect rules.
- Fix cookbook pages: keychain loading on every backend, received Close
  status, a multipart sample that trusted the client's file name, an
  echo sample that sent binary as text, SNI, pre-request handler scope,
  stop() with WebSocket handlers, and the thread pool notes.
- Restore the mutex in the LLM app's Chapter 5 code.

docs-src/pages/en/cookbook/s07-multipart-reader.md

@@ -48,22 +48,30 @@ svr.Post("/upload",
      const httplib::ContentReader &content_reader) {
     std::ofstream ofs;
 
-    content_reader(
+    auto ok = content_reader(
       [&](const httplib::FormData &file) {
-        if (!file.filename.empty()) {
-          ofs.open("uploads/" + file.filename, std::ios::binary);
-        }
-        return static_cast<bool>(ofs);
+        if (ofs.is_open()) { ofs.close(); }         // done with the previous file
+        if (file.filename.empty()) { return true; } // a plain field, skip it
+
+        auto name = httplib::sanitize_filename(file.filename);
+        if (name.empty()) { return false; }         // reject an unsafe name
+
+        ofs.open("uploads/" + name, std::ios::binary);
+        return ofs.is_open();
       },
       [&](const char *data, size_t len) {
+        if (!ofs.is_open()) { return true; }        // not part of a file
         ofs.write(data, len);
         return static_cast<bool>(ofs);
       });
 
+    if (!ok) { return; } // the status is already set
     res.set_content("uploaded", "text/plain");
   });
 ```
 
+The header callback runs at the start of every part, so it closes the previous file before opening the next one. `file.filename` is whatever the client sent, so always pass it through `httplib::sanitize_filename()`. An empty result means the name is unsafe.
+
 Only a small chunk sits in memory at any moment, so gigabyte-scale files are no problem.
 
 ## Count the parts yourself