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/cookie.ts

@@ -2,7 +2,7 @@ import { Response } from 'express';
 
 const COOKIE_NAME = 'trek_session';
 
-function cookieOptions(clear = false) {
+export function cookieOptions(clear = false) {
   const secure = process.env.COOKIE_SECURE !== 'false' && (process.env.NODE_ENV === 'production' || process.env.FORCE_HTTPS === 'true');
   return {
     httpOnly: true,