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/__tests__/error-handler.test.js

@@ -1,8 +1,18 @@
-jest.mock('../error-logger', () => ({
-  logError: jest.fn(),
+// Mock the unified logging module so we can verify logError is called
+// without writing to the actual error.log file
+jest.mock('../src/utils/logging', () => ({
+  logError: jest.fn().mockResolvedValue(),
+  safeErrorMessage: jest.fn((err) => {
+    if (!err) return 'An internal error occurred';
+    return err.message || String(err);
+  }),
+  createLogger: jest.fn(() => ({
+    info: jest.fn(), warn: jest.fn(), error: jest.fn(), debug: jest.fn()
+  })),
+  LOG_LEVELS: { debug: 0, info: 1, warn: 2, error: 3 }
 }));
 
-const { asyncHandler, errorMiddleware, notFoundHandler } = require('../error-handler');
+const { errorMiddleware, notFoundHandler } = require('../error-handler');
 const {
   AppError,
   ValidationError,
@@ -30,23 +40,6 @@ describe('Error Handler', () => {
     next = jest.fn();
   });
 
-  describe('asyncHandler', () => {
-    it('calls the wrapped function', async () => {
-      const fn = jest.fn().mockResolvedValue();
-      const wrapped = asyncHandler(fn);
-      await wrapped(req, res, next);
-      expect(fn).toHaveBeenCalledWith(req, res, next);
-    });
-
-    it('calls next(err) on rejected promise', async () => {
-      const error = new Error('async fail');
-      const fn = jest.fn().mockRejectedValue(error);
-      const wrapped = asyncHandler(fn);
-      await wrapped(req, res, next);
-      expect(next).toHaveBeenCalledWith(error);
-    });
-  });
-
   describe('errorMiddleware', () => {
     it('returns 400 for ValidationError', () => {
       const err = new ValidationError('bad input', 'email');