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

sonar-project.properties

@@ -0,0 +1,14 @@
+sonar.projectKey=TREK
+sonar.projectName=TREK
+sonar.sourceEncoding=UTF-8
+
+# Sources
+sonar.sources=client/src,server/src
+sonar.exclusions=**/node_modules/**,**/dist/**,**/build/**
+
+# Tests
+sonar.tests=server/tests
+sonar.test.inclusions=server/tests/**/*.ts
+
+# Coverage — path relative to repo root
+sonar.javascript.lcov.reportPaths=server/coverage/lcov.info