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

.github/workflows/test.yml

@@ -0,0 +1,41 @@
+name: Tests
+
+on:
+  push:
+    branches: [main, dev]
+    paths:
+      - 'server/**'
+      - '.github/workflows/test.yml'
+  pull_request:
+    branches: [main, dev]
+    paths:
+      - 'server/**'
+      - '.github/workflows/test.yml'
+
+jobs:
+  server-tests:
+    name: Server Tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: server/package-lock.json
+
+      - name: Install dependencies
+        run: cd server && npm ci
+
+      - name: Run tests
+        run: cd server && npm run test:coverage
+
+      - name: Upload coverage
+        if: success()
+        uses: actions/upload-artifact@v6
+        with:
+          name: coverage
+          path: server/coverage/
+          retention-days: 7