Add API endpoint reference table

[MyouzzZ]

Summary

• Add docs/api-endpoints.md with a table of the current FastAPI endpoints.
• Document request models, example requests/responses, common error responses, and in-memory state notes.
• Keep endpoint claims aligned with backend/main.py.

Closes #7.

Validation

• Documentation-only change; no runtime tests required.
• git diff --check -- docs/api-endpoints.md
• Checked every documented endpoint and error response against backend/main.py.