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/tests/setup.ts

@@ -0,0 +1,9 @@
+// Global test setup — runs before every test file.
+// Environment variables must be set before any module import so that
+// config.ts, database.ts, etc. pick them up at import time.
+
+// Fixed encryption key (64 hex chars = 32 bytes) for at-rest crypto in tests
+process.env.ENCRYPTION_KEY = 'a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6a7b8c9d0e1f2a3b4c5d6a7b8c9d0e1f2';
+process.env.NODE_ENV = 'test';
+process.env.COOKIE_SECURE = 'false';
+process.env.LOG_LEVEL = 'error'; // suppress info/debug logs in test output