[FEATURE]: Allow to configure which members should be assigned OpenCode Go subscriptions

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Describe the enhancement you want to request

Currently, OpenCode Go subscriptions are assigned to the account that subscribes (the paying account), but we want them to be assigned to specific members within a workspace.

In our organization, contract administrators and service users are clearly separated. For current operations, contract administrators prepare workspaces in advance where Go has been subscribed, then invite users as members. Specifically, we provide usage rights to users through the following workflow:

1. Create a Workspace for the intended user and subscribe to OpenCode Go
2. Invite the target user as a member to the Workspace and provide them with the administrator's API Key for use

While the above operation is generally functioning well, the following issues remain.

• Since the user has joined the Workspace with member privileges, they can't view their Go subscription usage by billing period even when logging into the Dashboard.
• The user cannot use OpenCode Go with their own API Key.

Allowing users to change which members receive Go subscriptions would resolve the above issue.We would be delighted if you could consider this.

No files, tests, or entry points are named in the issue. Begin by tracing workspace membership, Go subscription assignment, usage visibility, and API-key authorization; done means an administrator can select members to receive subscriptions and those members can view usage and use their own API keys.