Add backend health check troubleshooting notes

[MyouzzZ]

Summary

• Add docs/backend-health-troubleshooting.md for FastAPI backend health checks.
• Document expected root endpoint response, server/port/dependency/env failures, AI provider fallback, and CORS/browser blocking cases.
• Include a quick verification checklist for contributors.

Closes #8.

Validation

• Documentation-only change; no runtime tests required.
• git diff --check -- docs/backend-health-troubleshooting.md
• Checked expected response and health behavior against backend/main.py and SETUP_GUIDE.md.