Release/image tag inconsistency: v1.11.0 auth docs and Volcengine latest

## Summary

There are several inconsistencies between the latest GitHub release, the tagged documentation, and the published container tags:

1. GitHub's latest release is [v1.11.0](https://github.com/agent-infra/sandbox/releases/tag/v1.11.0).
2. The `v1.11.0` tagged README does not document `SANDBOX_API_KEY`.
3. `SANDBOX_API_KEY` was documented on `main` in #216, three days after the v1.11.0 release.
4. The published v1.11.0 image does support API key authentication.
5. GHCR `latest` currently resolves to v1.11.0, but the Volcengine mirror `latest` resolves to an older image reporting version `1.0.0.156`.

## Verified image metadata

Checked on 2026-08-20.

GHCR `latest` and the pinned v1.11.0 image currently resolve to this multi-platform digest:

`sha256:6328d7fd2f0ff0b4c147c3d05b3df1ce331f4a482eb6e550ecd64ed1fcf906e7`

The v1.11.0 AMD64 manifest is:

`sha256:9a597aaa3716aca2fd42a517ceedc41063e5ceedcef43eb68bf7c059c0128b7a`

The Volcengine mirror `latest` currently resolves to:

`sha256:742062f99915e5495df8d4bfeaf40a93197c87c7c47b4e2407cd2b6356df8f48`

Its AMD64 image reports version `1.0.0.156` and creation time `2025-12-08`, while the pinned Volcengine `1.11.0` tag resolves to the expected v1.11.0 image.

## Runtime verification

Using the published v1.11.0 AMD64 image with `SANDBOX_API_KEY` configured:

- Request without a key: HTTP 401
- Request with the correct key: HTTP 200
- API, VNC, and Code Server routes are protected
- Shell, file persistence, browser navigation, and screenshot endpoints work

This confirms that the v1.11.0 image supports the feature even though the tagged README does not document it.

## Why this is confusing

Users following different official image examples can receive different runtime capabilities depending on whether they use GHCR `latest`, Volcengine `latest`, or the pinned `1.11.0` tag.

Related reports include #160, #210, and #220. PR #217 pins the mainland-China example to `1.11.0`, which avoids the stale `latest` tag but does not clarify or repair the tag mapping itself.

## Request

Could the maintainers please clarify the intended release and image-tag policy and consider:

- documenting API key support in the v1.11.0 release notes;
- fixing the Volcengine `latest` tag;
- documenting whether `latest` is expected to track the latest GitHub release;
- recommending pinned release tags or digests for production deployments;
- optionally publishing a v1.11.1 patch release if a new immutable release is needed to synchronize the security documentation.

Thanks.