Consolidate response helpers and error logger to single modules

Two cleanups in one pass for the v1.14.0 'works on any platform' theme:

1. Response helpers — merged src/utils/responses.js and the root-level
   response-helpers.js into a single module at src/utils/responses.js.
   The old module had a richer set (created, noContent, validationError,
   unauthorized, forbidden, notFound, conflict) and is now re-exported
   from the new location. Updated 15 routes to import from
   src/utils/responses and deleted the root response-helpers.js.

2. Error logger — error-handler.js now uses the unified
   src/utils/logging.js#logError (same one src/app.js uses), so all errors
   go to one log file with one rotation policy. Removed the dead
   asyncHandler export (the real one is in src/utils/async-handler.js
   and is used everywhere). Deleted the legacy error-logger.js.

Both are invisible to users — same HTTP response shapes, same log file
path, same error format. Internal-only refactor.

dashcaddy-api/routes/config-drift.js

@@ -8,7 +8,7 @@
  */
 
 const express = require('express');
-const { success } = require('../response-helpers');
+const { success } = require('../src/utils/responses');
 const { ValidationError, NotFoundError } = require('../errors');
 
 /**