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/config.ts

@@ -1,6 +1,6 @@
-import crypto from 'crypto';
-import fs from 'fs';
-import path from 'path';
+import crypto from 'node:crypto';
+import fs from 'node:fs';
+import path from 'node:path';
 
 const dataDir = path.resolve(__dirname, '../data');