Add comprehensive backend test suite (#339)

* add test suite, mostly covers integration testing, tests are only backend side

* workflow runs the correct script

* workflow runs the correct script

* workflow runs the correct script

* unit tests incoming

* Fix multer silent rejections and error handler info leak

- Revert cb(null, false) to cb(new Error(...)) in auth.ts, collab.ts,
  and files.ts so invalid uploads return an error instead of silently
  dropping the file
- Error handler in app.ts now always returns 500 / "Internal server
  error" instead of forwarding err.message to the client

* Use statusCode consistently for multer errors and error handler

- Error handler in app.ts reads err.statusCode to forward the correct
  HTTP status while keeping the response body generic

server/src/services/notifications.ts

@@ -175,14 +175,14 @@ const EVENT_TEXTS: Record<string, Record<EventType, EventTextFn>> = {
 };
 
 // Get localized event text
-function getEventText(lang: string, event: EventType, params: Record<string, string>): EventText {
+export function getEventText(lang: string, event: EventType, params: Record<string, string>): EventText {
   const texts = EVENT_TEXTS[lang] || EVENT_TEXTS.en;
   return texts[event](params);
 }
 
 // ── Email HTML builder ─────────────────────────────────────────────────────
 
-function buildEmailHtml(subject: string, body: string, lang: string): string {
+export function buildEmailHtml(subject: string, body: string, lang: string): string {
   const s = I18N[lang] || I18N.en;
   const appUrl = getAppUrl();
   const ctaHref = appUrl || '#';
@@ -256,7 +256,7 @@ async function sendEmail(to: string, subject: string, body: string, userId?: num
   }
 }
 
-function buildWebhookBody(url: string, payload: { event: string; title: string; body: string; tripName?: string }): string {
+export function buildWebhookBody(url: string, payload: { event: string; title: string; body: string; tripName?: string }): string {
   const isDiscord = /discord(?:app)?\.com\/api\/webhooks\//.test(url);
   const isSlack = /hooks\.slack\.com\//.test(url);