Document API error response standard

[MyouzzZ]

Summary

• document the current FastAPI validation and HTTPException error behavior
• define a standard API error response schema with stable codes and examples
• list endpoints that should be updated and outline a backend/frontend migration plan

Closes #32.

Validation

• Reviewed backend/main.py validation models and explicit HTTPException paths
• Reviewed frontend non-OK response handling in study-copilot-full.html
• git diff --check -- docs/api-error-response-standard.md