Fix CI: cpplint include order in http_utils.cpp + Windows ERROR macro clash

Two unrelated CI regressions on PR #374, both falling out of TASK-020:

1. Lint job (gcc-14, ubuntu): cpplint flagged
   src/http_utils.cpp:30 with build/include_order, because the
   matching public header ("httpserver/http_utils.hpp") came AFTER a
   non-matching project header ("httpserver/constants.hpp"), and
   <microhttpd.h> (a C system header in cpplint's view) followed both.
   cpplint's expected order is: matching header, C system, C++ system,
   other. Reorder so the matching header comes first and the project
   headers ("constants.hpp" / "string_utilities.hpp") move to the
   bottom of the include block.

2. Windows MSYS2 build: src/httpserver/http_utils.hpp failed with
       error: expected identifier before numeric constant
   at the line `ERROR = 0,` inside the digest_auth_result enum.
   <wingdi.h> (pulled in via <windows.h> via <winsock2.h> via
   <microhttpd.h> on MinGW) unconditionally `#define`s ERROR to 0,
   and the preprocessor expands macros inside scoped-enum bodies just
   like anywhere else. Pre-TASK-020 the enum was inside
   `#ifdef HAVE_DAUTH`, so MSYS2 builds without digest auth never
   compiled it; PRD-FLG-REQ-001 then made the enum unconditional and
   exposed the latent collision. v2.0 is unreleased, so renaming is
   safe: ERROR -> GENERIC_ERROR (matches MHD_DAUTH_ERROR's "general
   error" docs). Static-assert pin in src/http_utils.cpp updated to
   match.

Verified locally:
  - python3 -m cpplint on both touched files: exit 0.
  - `make check` on macOS: 32/32 PASS, all check-hygiene /
    check-headers gates PASS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

src/http_utils.cpp

@@ -18,7 +18,6 @@
      USA
 */
 
-#include "httpserver/constants.hpp"
 #include "httpserver/http_utils.hpp"
 
 // TASK-020: <microhttpd.h> is no longer pulled in transitively by
@@ -57,6 +56,7 @@
 #include <utility>
 #include <vector>
 
+#include "httpserver/constants.hpp"
 #include "httpserver/string_utilities.hpp"
 
 #pragma GCC diagnostic ignored "-Warray-bounds"
@@ -634,8 +634,8 @@ static_assert(
     static_cast<int>(http_utils::digest_auth_result::OK) == MHD_DAUTH_OK,
     "digest_auth_result::OK diverged from MHD_DAUTH_OK");
 static_assert(
-    static_cast<int>(http_utils::digest_auth_result::ERROR) == MHD_DAUTH_ERROR,
-    "digest_auth_result::ERROR diverged from MHD_DAUTH_ERROR");
+    static_cast<int>(http_utils::digest_auth_result::GENERIC_ERROR) == MHD_DAUTH_ERROR,
+    "digest_auth_result::GENERIC_ERROR diverged from MHD_DAUTH_ERROR");
 static_assert(
     static_cast<int>(http_utils::digest_auth_result::WRONG_HEADER) == MHD_DAUTH_WRONG_HEADER,
     "digest_auth_result::WRONG_HEADER diverged from MHD_DAUTH_WRONG_HEADER");

src/httpserver/http_utils.hpp

@@ -152,7 +152,10 @@ class http_utils {
 
      enum class digest_auth_result {
          OK = 1,                  // MHD_DAUTH_OK
-         ERROR = 0,               // MHD_DAUTH_ERROR
+         // GENERIC_ERROR (not "ERROR") because <wingdi.h> on Windows
+         // unconditionally `#define`s ERROR to 0, which the preprocessor
+         // expands inside scoped-enum bodies just like anywhere else.
+         GENERIC_ERROR = 0,       // MHD_DAUTH_ERROR ("general error")
          WRONG_HEADER = -1,       // MHD_DAUTH_WRONG_HEADER
          WRONG_USERNAME = -2,     // MHD_DAUTH_WRONG_USERNAME
          WRONG_REALM = -3,        // MHD_DAUTH_WRONG_REALM