Milestone 0: import zappier billing, Verae middleware, and Zapier research

Compose-ready workspace: packages/zappier (rate card, portal, Stripe),
packages/verae-zapier-middleware (timestamp + NATS), packages/verae-zapier
(CLI app), vendor/zapier-platform, and research/zapier vendor corpus.

Gate 0 structure checks pass. Product code and research are not yet wired.

research/zapier/verae-zapier-api/verae-zapier-middleware/src/errors.js

@@ -0,0 +1,75 @@
+/**
+ * @fileoverview Structured errors and Express helpers.
+ * @module errors
+ */
+
+import { createDebugger } from './debug/logger.js';
+import { getTraceId } from './debug/trace-context.js';
+
+const log = createDebugger('app');
+
+/**
+ * Operational error with HTTP status and machine-readable code.
+ */
+export class AppError extends Error {
+  /**
+   * @param {string} message - Human-readable error.
+   * @param {object} [options]
+   * @param {number} [options.status=500] - HTTP status code.
+   * @param {string} [options.code='INTERNAL_ERROR'] - Machine code.
+   * @param {unknown} [options.details] - Optional structured details (safe for clients).
+   */
+  constructor(message, { status = 500, code = 'INTERNAL_ERROR', details } = {}) {
+    super(message);
+    this.name = 'AppError';
+    this.status = status;
+    this.code = code;
+    this.details = details;
+  }
+}
+
+/**
+ * Wrap an async Express handler so rejected promises reach the error middleware.
+ *
+ * @param {(req: import('express').Request, res: import('express').Response, next: import('express').NextFunction) => Promise<unknown>} fn
+ * @returns {import('express').RequestHandler}
+ */
+export function asyncHandler(fn) {
+  return (req, res, next) => {
+    Promise.resolve(fn(req, res, next)).catch(next);
+  };
+}
+
+/**
+ * Send a JSON error response. Logs server errors when debug is enabled.
+ *
+ * @param {import('express').Response} res
+ * @param {Error|AppError} err
+ * @returns {void}
+ */
+export function sendError(res, err) {
+  const status = err instanceof AppError ? err.status : 500;
+  const code = err instanceof AppError ? err.code : 'INTERNAL_ERROR';
+  const message = err?.message || 'Internal server error';
+  const details = err instanceof AppError ? err.details : undefined;
+  const traceId = getTraceId();
+
+  if (status >= 500) {
+    log.error('request failed', {
+      status,
+      code,
+      message,
+      traceId,
+      stack: err?.stack,
+    });
+  } else {
+    log.debug('client error', { status, code, message, traceId });
+  }
+
+  res.status(status).json({
+    error: message,
+    code,
+    ...(details !== undefined ? { details } : {}),
+    ...(traceId ? { traceId } : {}),
+  });
+}