Fix 9 security vulnerabilities found in audit

Add HAVE_BAUTH conditional compilation for basic authentication,
mirroring the existing HAVE_DAUTH pattern. This allows libhttpserver
to build against libmicrohttpd installations that lack basic auth
support.

Changes:
- configure.ac: Auto-detect MHD_queue_basic_auth_fail_response,
  define HAVE_BAUTH flag and AM_CONDITIONAL, add to summary output
- src/httpserver/basic_auth_fail_response.hpp: Guard with #ifdef HAVE_BAUTH
- src/basic_auth_fail_response.cpp: Guard with #ifdef HAVE_BAUTH
- src/httpserver.hpp: Conditionally include basic_auth_fail_response.hpp
- src/httpserver/http_request.hpp: Guard get_user(), get_pass(),
  fetch_user_pass() declarations and username/password cache fields
- src/http_request.cpp: Guard fetch_user_pass(), get_user(), get_pass()
  implementations and basic auth output in operator<<
- src/httpserver/create_webserver.hpp: Guard basic_auth()/no_basic_auth()
  methods and _basic_auth_enabled member
- src/httpserver/webserver.hpp: Guard basic_auth_enabled member
- src/webserver.cpp: Guard basic_auth_enabled initialization
- src/Makefile.am: Make basic_auth_fail_response conditional on HAVE_BAUTH
- examples/Makefile.am: Guard basic_authentication and
  centralized_authentication examples behind HAVE_BAUTH
- test/integ/authentication.cpp: Guard basic auth tests with HAVE_BAUTH
- test/unit/create_webserver_test.cpp: Guard basic_auth builder test

Guard basic auth references in ws_start_stop.cpp, basic.cpp,
create_test_request_test.cpp, create_test_request.hpp, and
create_test_request.cpp that would fail to compile when
libmicrohttpd lacks basic auth support.

Add Version 0.20.0 header with bauth conditional compilation and
security fix entries. Bump version in configure.ac to match.

Add HAVE_BAUTH conditional compilation for basic auth

The existing string_response already supports binary content (std::string
can hold arbitrary bytes), but this was not documented or demonstrated
anywhere. This gap caused users to believe a new response type was needed
(see PR #368).

- Add a note to the README's string_response description clarifying
  binary data support
- Add a new "Serving binary data from memory" section with inline example
- Add examples/binary_buffer_response.cpp as a complete, buildable example
  that serves a PNG image from an in-memory buffer
- Register the new example in examples/Makefile.am

https://claude.ai/code/session_01S3BvBrSoNvUhpYTyhPYCjJ

Add ChangeLog entry for the binary buffer example to satisfy the
ChangeLog Check workflow. Add missing #include <utility> for
std::move to fix cpplint warning.

https://claude.ai/code/session_01S3BvBrSoNvUhpYTyhPYCjJ

Add documentation and example for serving binary data from memory