Gate self-serve registration behind ENABLE_ACCOUNT_REGISTRATION (#25)

## Summary
- Add `ENABLE_ACCOUNT_REGISTRATION` (default `false`) for self-serve sign-up, set via env / control-node secret (`chat_backend_<env>.env`)
- Harden `POST /api/user/create/` to create company + user, return JWTs, and reject when the flag is off
- Expose `GET /api/public/settings/` with `enable_account_registration` for the SPA

Supports frontend: [chat_web_app#31](ai_ml_operations/chat_web_app#31)

## Test plan
- [ ] `cd llm_be && uv run python manage.py test chat_backend.tests.test_views_users`
- [ ] Confirm `POST /api/user/create/` returns 403 when flag unset/false
- [ ] With `ENABLE_ACCOUNT_REGISTRATION=true`, register returns access/refresh + creates company
- [ ] `GET /api/public/settings/` returns the flag
- [ ] Add `ENABLE_ACCOUNT_REGISTRATION=false` to control-node `chat_backend_prod.env` / `chat_backend_beta.env` before deploy (see `.env.prod.example`)Reviewed-on: #25

README.md

@@ -90,6 +90,7 @@ with `COMPOSE_DATABASE_URL` if needed.
 | `OLLAMA_MODEL` / `OLLAMA_EMBED_MODEL` | from `DEBUG` | optional | Override model names |
 | `EMAIL_HOST_*` | empty | yes (prod/beta) | SMTP2GO |
 | `CAPTCHA_SECRET_KEY` | empty | recommended | |
+| `ENABLE_ACCOUNT_REGISTRATION` | `false` | optional | Self-serve sign-up; keep false until ready |
 | `STRIPE_SECRET_KEY` / `STRIPE_PUBLISHABLE_KEY` / `STRIPE_WEBHOOK_SECRET` | empty | yes for billing | Stripe API + webhook |
 | `STRIPE_PRICE_ID` | empty | optional | Pre-created Price; else `$10/mo` from settings |
 | `FRONTEND_BASE_URL` | `http://localhost:3000` | set in prod | Checkout success/cancel base |