Support non-interactive Shopware Account linking and token refresh for partner-managed deployments

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User story

As a developer, I want to be able to maintain my Shopware Account connection to a shop without relying on regular Admin-based interaction, so that account-dependent functionality continues to work in partner-managed setups.

Context

For most partner-managed instances the plugin management is done on code-level (composer) and not within the admin. In many of these setups, users do not have access to the plugin-manager and the store plugin is not installed. Because they want to avoid having the user change the sourcecode on "runtime" via plugin downloads etc. For these scenarios, it is unlikely that a login will happen via the admin interface regularily. For some functionalities like in-app-purchases we need the login for license validation

Acceptance criteria

• A Shopware installation can be linked to a Shopware Account without using the Admin UI.
• The linking flow is suitable for CLI and CI/CD usage.
• Authentication can be refreshed or renewed non-interactively after expiry.
• The solution works even if the Store plugin is not installed.
• The solution supports partner-managed deployments where plugin management is done via Composer only.
• The documentation clearly explains the intended setup and operational model for headless environments.

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.