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/routes/collab.ts

@@ -43,7 +43,9 @@ const noteUpload = multer({
     const ext = path.extname(file.originalname).toLowerCase();
     const BLOCKED = ['.svg', '.html', '.htm', '.xml', '.xhtml', '.js', '.jsx', '.ts', '.exe', '.bat', '.sh', '.cmd', '.msi', '.dll', '.com', '.vbs', '.ps1', '.php'];
     if (BLOCKED.includes(ext) || file.mimetype.includes('svg') || file.mimetype.includes('html') || file.mimetype.includes('javascript')) {
-      return cb(new Error('File type not allowed'));
+      const err: Error & { statusCode?: number } = new Error('File type not allowed');
+      err.statusCode = 400;
+      return cb(err);
     }
     cb(null, true);
   },