Reduce Test Code Duplication

[baberlabs]

Problem:

Test files (especially backend/__tests__/message.test.js) contain massive amounts of duplicated setup code. The same user creation, authentication, and chat setup logic is repeated in every describe block.

Why it's an issue:

• Test files are 600+ lines with mostly boilerplate
• Changes to test setup require updates in multiple places
• Hard to maintain and understand actual test logic

Acceptance Criteria:

• Create shared test utilities for user setup and authentication
• Extract common fixtures into reusable functions
• Reduce test file length by at least 50%