Document frontend/backend integration contract

[MyouzzZ]

Summary

• document the frontend fetch calls and matching FastAPI endpoints
• list request payloads, response fields, and current frontend/backend mismatches
• add shared error response expectations and a follow-up contract checklist

Closes #39.

Validation

• Reviewed frontend fetch(...) calls in study-copilot-full.html
• Reviewed FastAPI route definitions and response fields in backend/main.py
• git diff --check -- docs/frontend-backend-contract.md