fix: improve test infrastructure and resolve mocking issues

- Fix Jest test runner configuration (was using bun test)
- Implement proper CouchDB service mocking in jest.preSetup.js
- Update errorhandling.test.js to use test app instead of real server
- Fix browserslist deprecation warnings
- Skip CouchDB initialization during test environment
- 22/22 Post model tests now passing
- 7/38 error handling tests now passing

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/__tests__/performance.test.js

@@ -1,5 +1,5 @@
 const request = require("supertest");
-const app = require("../server");
+const { app } = require("../server");
 const User = require("../models/User");
 const Street = require("../models/Street");
 const Task = require("../models/Task");