CodeRabbit Generated Unit Tests: Add Jest tests across app; add jest.config.js and update package.json

[coderabbitai]

Unit test generation was requested by @qwalski.

• adding new feature #1 (comment)

The following files were modified:

• jest.config.js
• package.json
• tests/config/database.test.js
• tests/controllers/activityController.test.js
• tests/controllers/todoController.test.js
• tests/routes/activityRoutes.test.js
• tests/server.test.js

[coderabbitai]

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