fix(backend): route every async handler through the error middleware (#59)

Express 4 does not forward a rejected promise from an async handler, so an unwrapped async route never responds at all — the request hangs until the client gives up, nothing reaches the error middleware, and monitoring sees an open connection rather than a 500. That silence is the shape of the 2026-08-17 incident, where an unhandled rejection left every item query hanging and the storefront rendered it as an empty shop. `asyncRoute` was written in response, but it was only applied to some routes: 30 handlers added afterwards were still bare, including register, login, the whole cart, and PayPal checkout.

Wraps all 30, plus two the issue's inventory missed. `attachCustomer` is a bare async middleware mounted globally in app.ts, so a rejection in its session lookup would hang every request in the application — including the 25 handlers that were already wrapped correctly, which meant the guarantee did not actually hold anywhere. The PayPal webhook registers on a second router named `webhookRouter`, so an audit grepping for `router.` walked straight past it.

Adds a unit test that scans the route sources and fails on any registration whose handler is not wrapped. A convention already half-forgotten once will be forgotten again, and enforcement is what the issue asked for; ESLint would be the better home for it but there is no ESLint in this repo yet (#60). The test walks parens rather than lines, so it also catches a handler whose `async` sits on its own line, and it matches any `*Router` name rather than just `router` — the two ways the existing bare handlers escaped notice. It is deleted along with `asyncRoute` if the project moves to Express 5, which forwards rejections natively.

No behaviour changes on the success path; the failure path turns a hung request into a logged 500.

Closes #59

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/routes/adminSettings.ts

@@ -1,18 +1,19 @@
 import { Router, Request, Response } from 'express';
 import { pool } from '../db';
+import { asyncRoute } from '../asyncRoute';
 
 const router = Router();
 
-router.get('/', async (_req: Request, res: Response) => {
+router.get('/', asyncRoute(async (_req: Request, res: Response) => {
   const { rows } = await pool.query(`SELECT key, value FROM admin_settings`);
   const map: Record<string, string> = {};
   for (const r of rows) map[r.key] = r.value;
   res.json({
     cartExpiryHours: parseFloat(map.cart_expiry_hours || '24')
   });
-});
+}));
 
-router.put('/', async (req: Request, res: Response) => {
+router.put('/', asyncRoute(async (req: Request, res: Response) => {
   const { cartExpiryHours } = req.body;
   const hours = parseFloat(cartExpiryHours);
   if (Number.isNaN(hours) || hours <= 0) {
@@ -24,6 +25,6 @@ router.put('/', async (req: Request, res: Response) => {
     [String(hours)]
   );
   res.json({ cartExpiryHours: hours });
-});
+}));
 
 export default router;