[REQ-a4-docs-08] A4 docs, .env.example, Postman, screenshots, demo checklist

[Yael-work]

README: purpose, install, DB setup, env vars, ORM setup, API endpoints, WebSocket feature, AI feature, known limitations. .env.example placeholders only. Postman: DB CRUD, AI analyze, reviewed-meal save, error examples. Screenshots: DB-connected app, CRUD, ORM relationships, WebSocket two clients, AI input/output, DB tables/migrations. Demo-video checklist. Write incrementally.