API/Error Handling QA

PROJECT CONTEXT:
I want my app's API layer verified: correct behavior on success, and defined, handled behavior on every failure.

GOAL:
Test every endpoint's contract and failure modes, and verify the frontend handles each failure visibly and recoverably.

REQUIREMENTS:
- An endpoint inventory with expected contracts
- Status code and error shape verification per endpoint
- Hostile input and edge testing on every route
- Frontend handling verified for each failure class
- Timeout and retry behavior defined and tested

CONSTRAINTS:
- Test with real requests, not code review
- Every failure class must be visible in the UI

DEFINITION OF DONE:
- Every endpoint returns correct codes for success and failures
- Error responses share a consistent, safe shape
- The frontend shows appropriate UI per failure class
- Slow responses hit timeouts with recoverable UX

COMMON FAILURES TO AVOID:
- 200 responses containing error strings
- 500s from bad user input instead of 400s
- Different error shapes per endpoint breaking the frontend
- No timeouts, so slow APIs hang the UI forever